grpcproxy: invalidate comparison keys after txn
grpcproxy: invalidate comparison keys after txn

If the txn comparison block makes claims about a key's current
state, then it may say a key has been updated. Future range/txn
operations may expect this update to eventually be propagated through
the cluster and show up in serialized requests. To avoid spinning
forever on txn/serialized range loops, invalidate the comparison keys.

hongchaodeng/etcd,qqsun8819/etcd,lumjjb/etcd,mickep76/etcd,OyTao/etcd_learning,heyitsanthony/etcd,YuleiXiao/etcd,mqliang/etcd,visheshnp/etcd,li-ang/etcd,dannysauer/etcd,jlamillan/etcd,armstrongli/etcd,yudai/etcd,lorneli/etcd,rnaveiras/etcd,lorneli/etcd,a-robinson/etcd,bluebreezecf/etcd,dannysauer/etcd,plasticbox/etcd,etcd-io/etcd,CrazyJvm/etcd,xiang90/etcd,ravigadde/etcd,whuwxl/etcd,fanminshi/etcd,glevand/coreos--etcd,hongchaodeng/etcd,fanminshi/etcd,sinsharat/etcd,heyitsanthony/etcd,cockroachdb/etcd,YuleiXiao/etcd,whuwxl/etcd,mqliang/etcd,ajityagaty/etcd,akauppi/etcd,coreos/etcd,armstrongli/etcd,bdarnell/etcd,gyuho/etcd,etcd-io/etcd,handsomesun/etcd,mkumatag/etcd,rnaveiras/etcd,OyTao/etcd_learning,mickep76/etcd,insequent/etcd,insequent/etcd,akauppi/etcd,glevand/coreos--etcd,ajityagaty/etcd,a-robinson/etcd,tguangch/etcd,qqsun8819/etcd,sebrandon1/etcd,rchicoli/etcd,endocode/etcd,tguangch/etcd,sinsharat/etcd,cockroachdb/etcd,sebrandon1/etcd,vimalk78/etcd,bdarnell/etcd,xiang90/etcd,rchicoli/etcd,ajityagaty/etcd,etcd-io/etcd,ravigadde/etcd,vimalk78/etcd,li-ang/etcd,joshix/etcd,rnaveiras/etcd,jlamillan/etcd,coreos/etcd,yudai/etcd,mqliang/etcd,endocode/etcd,handsomesun/etcd,plasticbox/etcd,mkumatag/etcd,joshix/etcd,gyuho/etcd,lumjjb/etcd,visheshnp/etcd,bluebreezecf/etcd,CrazyJvm/etcd,mkumatag/etcd