Add instructions for setting a value only if the key does not exist
Add instructions for setting a value only if the key does not exist

Use an empty prevValue to atomically test for the existence of a key before setting the value.
cvik/etcd,junxu/etcd,qqsun8819/etcd,sebschrader/etcd,sofuture/etcd,fasaxc/etcd,ylong/etcd,imzhulei/etcd,rchicoli/etcd,etcd-io/etcd,spacejam/etcd,asim/etcd,spotify/etcd,mwitkow-io/etcd,cloudfoundry/etcd,arthurcolle/etcd,mike-lerch/etcd,dannysauer/etcd,junxu/etcd,openedbox/etcd,shafreeck/etcd,gae123/etcd,QuantiumTechnology/etcd,insequent/etcd,elimisteve/etcd,armstrongli/etcd,fgbreel/etcd,gosharplite/etcd,pigfoot/etcd,spotify/etcd,pengzj/etcd,barakmich/etcd,cvik/etcd,cchamplin/etcd,gouyang/etcd,Ulexus/etcd,iamqizhao/etcd,sebrandon1/etcd,whuwxl/etcd,fanminshi/etcd,jlamillan/etcd,hongbinz/etcd,glensc/etcd,westhood/etcd,shawnps/etcd,shakefu/etcd,jonboulle/etcd,yifan-gu/etcd,rnaveiras/etcd,visheshnp/etcd,xdlinear/etcd,mischief/etcd,chipironcin/etcd,gtank/etcd,svdata/etcd,giantswarm/etcd,xychu/etcd,yifan-gu/etcd,qiaochao911/etcd,jefby/etcd,linuxcer/etcd,ravigadde/etcd,pauldallen/etcd,wantonsolutions/dinv-etcd,sofuture/etcd,yudai/etcd,devfreesg/etcd,Winslett/etcd,CrazyJvm/etcd,gityf/etcd,snailwalker/etcd,nkhare/etcd,yichengq/etcd,mkumatag/etcd,imzhulei/etcd,TheHippo/etcd,nekto0n/etcd,achanda/etcd,es-chow/etcd,gityf/etcd,bdarnell/etcd,jchauncey/etcd,Ulexus/etcd,StorReduce/etcd,yichengq/etcd,heyitsanthony/etcd,zaphod-concur/etcd,axingblog/etcd,tamird/etcd,pengzj/etcd,insequent/etcd,wowsoso/etcd,handsomesun/etcd,spotify/etcd,jlamillan/etcd,Ulexus/etcd,SivagnanamCiena/etcd,cchamplin/etcd,dannysauer/etcd,HideoYamauchi/etcd,wfxiang08/etcd,steveeJ/etcd,justintung/etcd,xiang90/etcd,njucslqq/etcd,steveeJ/etcd,li-ang/etcd,beni55/etcd,siddontang/etcd,mwitkow/etcd,plasticbox/etcd,raoofm/etcd,tguangch/etcd,cvik/etcd,jmptrader/etcd,kingland/etcd,youxidev/etcd,wfxiang08/etcd,nieanan/etcd,mischief/etcd,delkyd/etcd,spacejam/etcd,dalanlan/etcd,gtank/etcd,linuxcer/etcd,msanterre/etcd,OyTao/etcd_learning,Ulexus/etcd,ajityagaty/etcd,mgoelzer/etcd-fork2,glevand/coreos--etcd,rnaveiras/etcd,mitake/etcd,xdlinear/etcd,mickep76/etcd,polvi/etcd,glycerine/etcd,QuantiumTechnology/etcd,improbable-io/etcd,xebitstudios/etcd,hroyrh/etcd,njucslqq/etcd,jravetch/etcd,tguangch/etcd,joshix/etcd,svdata/etcd,endocode/etcd,princeqjzh/etcd,dterei/etcd,lzbgt/etcd,yyljlyy/etcd,sebrandon1/etcd,bakins/etcd,tangkun75/etcd,OyTao/etcd_learning,chai2010-playground/etcd,spotify/etcd,wantonsolutions/dinv-etcd,es-chow/etcd,rnaveiras/etcd,shakefu/etcd,luxas/etcd,jefby/etcd,barakmich/etcd,njharman/etcd,mqliang/etcd,giantswarm/etcd,HideoYamauchi/etcd,hpcloud/etcd,freeloop1114/etcd,yyljlyy/etcd,cloudfoundry/etcd,freeloop1114/etcd,TheHippo/etcd,olark/etcd,mqliang/etcd,joshix/etcd,combk8s/etcd,glycerine/etcd,xueshanf/etcd,chai2010/etcd,olark/etcd,Timer/etcd,plasticbox/etcd,chipironcin/etcd,princeqjzh/etcd,dnaeon/etcd,chenbaihu/etcd,chenbaihu/etcd,Yhgenomics/etcd,sayden/etcd,offscale/etcd,tamird/etcd,oscarrr110/etcd,vimalk78/etcd,cognusion/etcd,justintung/etcd,fasaxc/etcd,lumjjb/etcd,chai2010-playground/etcd,honeyqa/etcd,msanterre/etcd,siddontang/etcd,cockroachdb/etcd,fasaxc/etcd,mwitkow/etcd,yudai/etcd,luxas/etcd,hzy001/etcd,nieanan/etcd,jmptrader/etcd,buaazp/etcd,insionng/etcd,gyuho/etcd,delkyd/etcd,crandles/etcd,jeremyeder/etcd,nkhare/etcd,xebitstudios/etcd,YuleiXiao/etcd,mkumatag/etcd,iamqizhao/etcd,cloudfoundry/etcd,dnaeon/etcd,polvi/etcd,csuideal/etcd,elimisteve/etcd,etcd-io/etcd,StorReduce/etcd,mgoelzer/etcd-fork2,jravetch/etcd,lumjjb/etcd,xychu/etcd,bspline/etcd,mitake/etcd,crandles/etcd,lzbgt/etcd,tangkun75/etcd,li-ang/etcd,achanda/etcd,qqsun8819/etcd,Timer/etcd,gosharplite/etcd,vimalk78/etcd,offscale/etcd,ecnahc515/etcd,wantonsolutions/dinv-etcd,cognusion/etcd,hpcloud/etcd,honeyqa/etcd,purpleidea/etcd,combk8s/etcd,hongbinz/etcd,Eagle-X/etcd,jonboulle/etcd,ecnahc515/etcd,lorneli/etcd,shafreeck/etcd,openedbox/etcd,ajityagaty/etcd,youxidev/etcd,kkaneda/etcd,akauppi/etcd,xueshanf/etcd,robszumski/etcd,armstrongli/etcd,whuwxl/etcd,swingbach/etcd,swingbach/etcd,axingblog/etcd,dalanlan/etcd,sinsharat/etcd,gae123/etcd,beni55/etcd,nekto0n/etcd,pauldallen/etcd,bspline/etcd,arthurcolle/etcd,a-robinson/etcd,ktateish/etcd,glensc/etcd,bluebreezecf/etcd,shawnps/etcd,rchicoli/etcd,Eagle-X/etcd,raoofm/etcd,westhood/etcd,coreos/etcd,robszumski/etcd,buaazp/etcd,gyuho/etcd,cockroachdb/etcd,etcd-io/etcd,betawaffle/etcd,coreos/etcd,chai2010/etcd,asim/etcd,glevand/coreos--etcd,ylong/etcd,ravigadde/etcd,YuleiXiao/etcd,tempbottle/etcd,onlyjob/etcd,a-robinson/etcd,hongchaodeng/etcd,cvik/etcd,zaphod-concur/etcd,mike-lerch/etcd,ajityagaty/etcd,heyitsanthony/etcd,mwitkow-io/etcd,bakins/etcd,lorneli/etcd,wowsoso/etcd,qiaochao911/etcd,jeremyeder/etcd,SivagnanamCiena/etcd,hongchaodeng/etcd,hroyrh/etcd,cloudfoundry/etcd,tempbottle/etcd,improbable-io/etcd,gouyang/etcd,handsomesun/etcd,ktateish/etcd,insionng/etcd,akauppi/etcd,sayden/etcd,snailwalker/etcd,mqliang/etcd,kingland/etcd,oscarrr110/etcd,mickep76/etcd,bdarnell/etcd,sebschrader/etcd,endocode/etcd,hzy001/etcd,visheshnp/etcd,csuideal/etcd,Yhgenomics/etcd,xiang90/etcd,devfreesg/etcd,njharman/etcd,onlyjob/etcd,Winslett/etcd,bluebreezecf/etcd,betawaffle/etcd,fanminshi/etcd,sinsharat/etcd,dterei/etcd,jchauncey/etcd,mkumatag/etcd,fgbreel/etcd,CrazyJvm/etcd,purpleidea/etcd,kkaneda/etcd