clientv3: synchronize on goroutines in TestBalancerDoNotBlockOnClose
clientv3: synchronize on goroutines in TestBalancerDoNotBlockOnClose

Was leaking dialers.

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