[API] Timer-start behavior (bug)fix
[API] Timer-start behavior (bug)fix

Only start the timer when it is not running currently. In old behavior
the current slice-time was discarded, now multiple starts have no
effect.

catiedev/mbed-os,mikaleppanen/mbed-os,j-greffe/mbed-os,autopulated/mbed,pedromes/mbed,mazimkhan/mbed-os,fpiot/mbed-ats,mazimkhan/mbed-os,Archcady/mbed-os,fpiot/mbed-ats,adamgreen/mbed,maximmbed/mbed,nvlsianpu/mbed,tung7970/mbed-os,mikaleppanen/mbed-os,Sweet-Peas/mbed,pbrook/mbed,getopenmono/mbed,FranklyDev/mbed,getopenmono/mbed,ARM-software/mbed-beetle,mikaleppanen/mbed-os,ryankurte/mbed-os,sam-geek/mbed,dbestm/mbed,theotherjimmy/mbed,svastm/mbed,fpiot/mbed-ats,bremoran/mbed-drivers,xcrespo/mbed,jrjang/mbed,kpurusho/mbed,kpurusho/mbed,alertby/mbed,fanghuaqi/mbed,ban4jp/mbed,K4zuki/mbed,mmorenobarm/mbed-os,Shengliang/mbed,autopulated/mbed,kl-cruz/mbed-os,Willem23/mbed,screamerbg/mbed,kjbracey-arm/mbed,Sweet-Peas/mbed,bulislaw/mbed-os,andreaslarssonublox/mbed,nRFMesh/mbed-os,bikeNomad/mbed,mnlipp/mbed,ban4jp/mbed,mmorenobarm/mbed-os,wodji/mbed,dbestm/mbed,pi19404/mbed,dbestm/mbed,fahhem/mbed-os,monkiineko/mbed-os,hwfwgrp/mbed,autopulated/mbed,brstew/MBED-BUILD,jpbrucker/mbed,CalSol/mbed,svogl/mbed-os,Sweet-Peas/mbed,c1728p9/mbed-os,jamesadevine/mbed,nRFMesh/mbed-os,arostm/mbed-os,kpurusho/mbed,logost/mbed,jpbrucker/mbed,andreaslarssonublox/mbed,al177/mbed,jamesadevine/mbed,FranklyDev/mbed,pbrook/mbed,K4zuki/mbed,adustm/mbed,karsev/mbed-os,pbrook/mbed,rosterloh/mbed,theotherjimmy/mbed,logost/mbed,pi19404/mbed,fvincenzo/mbed-os,RonEld/mbed,fanghuaqi/mbed,al177/mbed,Sweet-Peas/mbed,tung7970/mbed-os-1,bentwire/mbed,pedromes/mbed,sg-/mbed-drivers,YarivCol/mbed-os,HeadsUpDisplayInc/mbed,netzimme/mbed-os,nabilbendafi/mbed,geky/mbed,wodji/mbed,mikaleppanen/mbed-os,svogl/mbed-os,bcostm/mbed-os,Marcomissyou/mbed,al177/mbed,svogl/mbed-os,struempelix/mbed,andcor02/mbed-os,mbedmicro/mbed,mnlipp/mbed,naves-thiago/mbed-midi,jpbrucker/mbed,JasonHow44/mbed,DanKupiniak/mbed,bikeNomad/mbed,screamerbg/mbed,Shengliang/mbed,al177/mbed,tung7970/mbed-os,K4zuki/mbed,wodji/mbed,kjbracey-arm/mbed,kpurusho/mbed,mbedmicro/mbed,Archcady/mbed-os,tung7970/mbed-os-1,hwfwgrp/mbed,Marcomissyou/mbed,naves-thiago/mbed-midi,naves-thiago/mbed-midi,struempelix/mbed,arostm/mbed-os,fahhem/mbed-os,autopulated/mbed,fahhem/mbed-os,RonEld/mbed,screamerbg/mbed,NXPmicro/mbed,svogl/mbed-os,YarivCol/mbed-os,Archcady/mbed-os,alertby/mbed,mazimkhan/mbed-os,YarivCol/mbed-os,jrjang/mbed,kl-cruz/mbed-os,fpiot/mbed-ats,karsev/mbed-os,geky/mbed,xcrespo/mbed,pradeep-gr/mbed-os5-onsemi,arostm/mbed-os,devanlai/mbed,rgrover/mbed,adustm/mbed,struempelix/mbed,bcostm/mbed-os,wodji/mbed,bulislaw/mbed-os,nvlsianpu/mbed,masaohamanaka/mbed,fvincenzo/mbed-os,xcrespo/mbed,andreaslarssonublox/mbed,radhika-raghavendran/mbed-os5.1-onsemi,svastm/mbed,pi19404/mbed,andcor02/mbed-os,kl-cruz/mbed-os,Willem23/mbed,brstew/MBED-BUILD,hwfwgrp/mbed,ban4jp/mbed,jferreir/mbed,masaohamanaka/mbed,radhika-raghavendran/mbed-os5.1-onsemi,jeremybrodt/mbed,kpurusho/mbed,jferreir/mbed,bremoran/mbed-drivers,maximmbed/mbed,logost/mbed,HeadsUpDisplayInc/mbed,jrjang/mbed,bulislaw/mbed-os,ARM-software/mbed-beetle,Shengliang/mbed,monkiineko/mbed-os,maximmbed/mbed,karsev/mbed-os,adustm/mbed,sam-geek/mbed,pbrook/mbed,mnlipp/mbed,hwfwgrp/mbed,jrjang/mbed,struempelix/mbed,struempelix/mbed,NitinBhaskar/mbed,al177/mbed,mmorenobarm/mbed-os,sam-geek/mbed,rgrover/mbed,jeremybrodt/mbed,catiedev/mbed-os,bcostm/mbed-os,mbedmicro/mbed,masaohamanaka/mbed,fanghuaqi/mbed,GustavWi/mbed,fahhem/mbed-os,Willem23/mbed,YarivCol/mbed-os,Timmmm/mbed,getopenmono/mbed,nvlsianpu/mbed,pbrook/mbed,ban4jp/mbed,kl-cruz/mbed-os,jeremybrodt/mbed,j-greffe/mbed-os,pi19404/mbed,EmuxEvans/mbed,rgrover/mbed,karsev/mbed-os,jamesadevine/mbed,ban4jp/mbed,mnlipp/mbed,bcostm/mbed-os,j-greffe/mbed-os,rosterloh/mbed,karsev/mbed-os,wodji/mbed,jamesadevine/mbed,svastm/mbed,0xc0170/mbed-drivers,alertby/mbed,NitinBhaskar/mbed,bikeNomad/mbed,maximmbed/mbed,monkiineko/mbed-os,jrjang/mbed,mikaleppanen/mbed-os,screamerbg/mbed,naves-thiago/mbed-midi,FranklyDev/mbed,mbedmicro/mbed,nabilbendafi/mbed,pedromes/mbed,screamerbg/mbed,bentwire/mbed,catiedev/mbed-os,Willem23/mbed,NXPmicro/mbed,Willem23/mbed,andcor02/mbed-os,NXPmicro/mbed,tung7970/mbed-os,Tiryoh/mbed,ryankurte/mbed-os,NXPmicro/mbed,svogl/mbed-os,fahhem/mbed-os,radhika-raghavendran/mbed-os5.1-onsemi,fvincenzo/mbed-os,mazimkhan/mbed-os,nabilbendafi/mbed,jferreir/mbed,monkiineko/mbed-os,rosterloh/mbed,mazimkhan/mbed-os,xcrespo/mbed,K4zuki/mbed,pradeep-gr/mbed-os5-onsemi,mikaleppanen/mbed-os,GustavWi/mbed,ryankurte/mbed-os,EmuxEvans/mbed,brstew/MBED-BUILD,jpbrucker/mbed,jpbrucker/mbed,Marcomissyou/mbed,arostm/mbed-os,Marcomissyou/mbed,infinnovation/mbed-os,autopulated/mbed,Timmmm/mbed,nabilbendafi/mbed,jamesadevine/mbed,Timmmm/mbed,HeadsUpDisplayInc/mbed,j-greffe/mbed-os,jeremybrodt/mbed,al177/mbed,xcrespo/mbed,theotherjimmy/mbed,cvtsi2sd/mbed-os,betzw/mbed-os,bcostm/mbed-os,wodji/mbed,pradeep-gr/mbed-os5-onsemi,infinnovation/mbed-os,K4zuki/mbed,infinnovation/mbed-os,catiedev/mbed-os,ryankurte/mbed-os,dbestm/mbed,tung7970/mbed-os,masaohamanaka/mbed,adamgreen/mbed,RonEld/mbed,fpiot/mbed-ats,betzw/mbed-os,naves-thiago/mbed-midi,autopulated/mbed,fvincenzo/mbed-os,alertby/mbed,Tiryoh/mbed,ARM-software/mbed-beetle,nabilbendafi/mbed,devanlai/mbed,fvincenzo/mbed-os,c1728p9/mbed-os,svastm/mbed,tung7970/mbed-os-1,EmuxEvans/mbed,FranklyDev/mbed,Sweet-Peas/mbed,radhika-raghavendran/mbed-os5.1-onsemi,larks/mbed,bikeNomad/mbed,nRFMesh/mbed-os,larks/mbed,GustavWi/mbed,nRFMesh/mbed-os,netzimme/mbed-os,jferreir/mbed,pedromes/mbed,larks/mbed,logost/mbed,dbestm/mbed,bulislaw/mbed-os,geky/mbed,hwfwgrp/mbed,theotherjimmy/mbed,Marcomissyou/mbed,brstew/MBED-BUILD,devanlai/mbed,bcostm/mbed-os,CalSol/mbed,netzimme/mbed-os,EmuxEvans/mbed,theotherjimmy/mbed,HeadsUpDisplayInc/mbed,Archcady/mbed-os,kjbracey-arm/mbed,kl-cruz/mbed-os,adamgreen/mbed,andcor02/mbed-os,DanKupiniak/mbed,adustm/mbed,rgrover/mbed,rosterloh/mbed,pradeep-gr/mbed-os5-onsemi,j-greffe/mbed-os,catiedev/mbed-os,maximmbed/mbed,pedromes/mbed,netzimme/mbed-os,tung7970/mbed-os-1,ryankurte/mbed-os,cvtsi2sd/mbed-os,Tiryoh/mbed,hwfwgrp/mbed,Shengliang/mbed,fpiot/mbed-ats,karsev/mbed-os,pradeep-gr/mbed-os5-onsemi,ryankurte/mbed-os,svastm/mbed,YarivCol/mbed-os,Timmmm/mbed,kl-cruz/mbed-os,andreaslarssonublox/mbed,nvlsianpu/mbed,c1728p9/mbed-os,rosterloh/mbed,arostm/mbed-os,pedromes/mbed,c1728p9/mbed-os,radhika-raghavendran/mbed-os5.1-onsemi,sam-geek/mbed,adustm/mbed,GustavWi/mbed,dbestm/mbed,geky/mbed,andcor02/mbed-os,K4zuki/mbed,brstew/MBED-BUILD,JasonHow44/mbed,theotherjimmy/mbed,getopenmono/mbed,JasonHow44/mbed,jferreir/mbed,0xc0170/mbed-drivers,Marcomissyou/mbed,alertby/mbed,logost/mbed,betzw/mbed-os,DanKupiniak/mbed,RonEld/mbed,tung7970/mbed-os-1,getopenmono/mbed,cvtsi2sd/mbed-os,screamerbg/mbed,GustavWi/mbed,iriark01/mbed-drivers,larks/mbed,FranklyDev/mbed,bentwire/mbed,rosterloh/mbed,mmorenobarm/mbed-os,mmorenobarm/mbed-os,ban4jp/mbed,struempelix/mbed,j-greffe/mbed-os,Tiryoh/mbed,c1728p9/mbed-os,adamgreen/mbed,DanKupiniak/mbed,bentwire/mbed,pi19404/mbed,cvtsi2sd/mbed-os,EmuxEvans/mbed,iriark01/mbed-drivers,naves-thiago/mbed-midi,andcor02/mbed-os,Sweet-Peas/mbed,brstew/MBED-BUILD,devanlai/mbed,netzimme/mbed-os,adamgreen/mbed,Tiryoh/mbed,fanghuaqi/mbed,mnlipp/mbed,nvlsianpu/mbed,mmorenobarm/mbed-os,tung7970/mbed-os,jpbrucker/mbed,svogl/mbed-os,cvtsi2sd/mbed-os,logost/mbed,infinnovation/mbed-os,betzw/mbed-os,catiedev/mbed-os,YarivCol/mbed-os,bikeNomad/mbed,nRFMesh/mbed-os,adamgreen/mbed,infinnovation/mbed-os,fanghuaqi/mbed,NitinBhaskar/mbed,c1728p9/mbed-os,HeadsUpDisplayInc/mbed,masaohamanaka/mbed,jeremybrodt/mbed,masaohamanaka/mbed,CalSol/mbed,rgrover/mbed,betzw/mbed-os,pradeep-gr/mbed-os5-onsemi,xcrespo/mbed,monkiineko/mbed-os,nabilbendafi/mbed,nvlsianpu/mbed,NXPmicro/mbed,NXPmicro/mbed,RonEld/mbed,ARM-software/mbed-beetle,nRFMesh/mbed-os,getopenmono/mbed,JasonHow44/mbed,jferreir/mbed,devanlai/mbed,devanlai/mbed,betzw/mbed-os,JasonHow44/mbed,adustm/mbed,bulislaw/mbed-os,alertby/mbed,cvtsi2sd/mbed-os,larks/mbed,pbrook/mbed,fahhem/mbed-os,kjbracey-arm/mbed,infinnovation/mbed-os,Archcady/mbed-os,Archcady/mbed-os,kpurusho/mbed,EmuxEvans/mbed,andreaslarssonublox/mbed,HeadsUpDisplayInc/mbed,larks/mbed,Shengliang/mbed,radhika-raghavendran/mbed-os5.1-onsemi,CalSol/mbed,sg-/mbed-drivers,bulislaw/mbed-os,NitinBhaskar/mbed,RonEld/mbed,Timmmm/mbed,jamesadevine/mbed,netzimme/mbed-os,arostm/mbed-os,NitinBhaskar/mbed,mbedmicro/mbed,Timmmm/mbed,maximmbed/mbed,bentwire/mbed,mazimkhan/mbed-os,CalSol/mbed,mnlipp/mbed,sam-geek/mbed,monkiineko/mbed-os,JasonHow44/mbed,bentwire/mbed,Tiryoh/mbed,geky/mbed,CalSol/mbed,Shengliang/mbed,jrjang/mbed,pi19404/mbed