clientv3: fix race on writing watch channel over return channel
clientv3: fix race on writing watch channel over return channel

Found in TestElectionFailover

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