clientv3/concurrency: don't poll in mutex lock on early prior key delete
clientv3/concurrency: don't poll in mutex lock on early prior key delete

Lock would get the prior key on retry using WithRev(myRev - 1) instead of
using the latest revision; the Watch() would return immediately and Lock
devolves into polling.

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