grpcproxy: fix deadlock in watchbroadcast
grpcproxy: fix deadlock in watchbroadcast

Calling empty() in watchbroadcast methods was trying to
lock the rwmutex when it was already held.

Fixes #6937

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