etcdserver: start real txn for txn request
etcdserver: start real txn for txn request

We should open real txn for applying txn requests. Or the intermediate
state might be observed by reader.

This also fixes #3803. Same consistent(raft) index per multiple indenpendent
operations confuses consistentStore.

cockroachdb/etcd,chipironcin/etcd,mickep76/etcd,sinsharat/etcd,YuleiXiao/etcd,dnaeon/etcd,lorneli/etcd,li-ang/etcd,akauppi/etcd,heyitsanthony/etcd,oscarrr110/etcd,raoofm/etcd,mqliang/etcd,tguangch/etcd,wantonsolutions/dinv-etcd,rnaveiras/etcd,dnaeon/etcd,yudai/etcd,luxas/etcd,plasticbox/etcd,qqsun8819/etcd,plasticbox/etcd,insequent/etcd,gyuho/etcd,tamird/etcd,sinsharat/etcd,cchamplin/etcd,chipironcin/etcd,siddontang/etcd,sebrandon1/etcd,a-robinson/etcd,fanminshi/etcd,lumjjb/etcd,Timer/etcd,whuwxl/etcd,zaphod-concur/etcd,westhood/etcd,rnaveiras/etcd,wantonsolutions/dinv-etcd,ajityagaty/etcd,siddontang/etcd,rchicoli/etcd,armstrongli/etcd,westhood/etcd,dannysauer/etcd,fasaxc/etcd,xiang90/etcd,coreos/etcd,StorReduce/etcd,mqliang/etcd,purpleidea/etcd,rchicoli/etcd,nekto0n/etcd,bluebreezecf/etcd,StorReduce/etcd,ajityagaty/etcd,luxas/etcd,handsomesun/etcd,li-ang/etcd,joshix/etcd,vimalk78/etcd,akauppi/etcd,glycerine/etcd,insequent/etcd,handsomesun/etcd,mickep76/etcd,cockroachdb/etcd,ravigadde/etcd,coreos/etcd,ktateish/etcd,nekto0n/etcd,purpleidea/etcd,wantonsolutions/dinv-etcd,hongchaodeng/etcd,HideoYamauchi/etcd,OyTao/etcd_learning,glycerine/etcd,endocode/etcd,swingbach/etcd,etcd-io/etcd,visheshnp/etcd,linuxcer/etcd,ktateish/etcd,fanminshi/etcd,HideoYamauchi/etcd,glevand/coreos--etcd,oscarrr110/etcd,sayden/etcd,shawnps/etcd,jonboulle/etcd,CrazyJvm/etcd,glevand/coreos--etcd,achanda/etcd,qqsun8819/etcd,hongchaodeng/etcd,heyitsanthony/etcd,etcd-io/etcd,armstrongli/etcd,jonboulle/etcd,whuwxl/etcd,xiang90/etcd,vimalk78/etcd,sebrandon1/etcd,swingbach/etcd,mkumatag/etcd,lorneli/etcd,linuxcer/etcd,a-robinson/etcd,tamird/etcd,joshix/etcd,sayden/etcd,tguangch/etcd,ajityagaty/etcd,visheshnp/etcd,mqliang/etcd,bluebreezecf/etcd,lumjjb/etcd,OyTao/etcd_learning,mkumatag/etcd,bdarnell/etcd,endocode/etcd,cchamplin/etcd,Timer/etcd,etcd-io/etcd,raoofm/etcd,rnaveiras/etcd,sofuture/etcd,gyuho/etcd,shawnps/etcd,dannysauer/etcd,jlamillan/etcd,achanda/etcd,fasaxc/etcd,CrazyJvm/etcd,ravigadde/etcd,mkumatag/etcd,bdarnell/etcd,sofuture/etcd,fasaxc/etcd,yudai/etcd,zaphod-concur/etcd,YuleiXiao/etcd,jlamillan/etcd