grpcproxy: lock store when getting size
grpcproxy: lock store when getting size

Fixes data race in proxy integration tests.

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