grpcproxy: support forcing leader as available
grpcproxy: support forcing leader as available

Leadership timeout can sometimes take too long, such as in test cases.
However, it is possible to infer a leader is available based on RPCs
that must go through consensus. Therefore, have a way to update the
leadership status off the watch path.

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