clientv3: do not mark stale endpoints as unhealthy
clientv3: do not mark stale endpoints as unhealthy

gyuho/etcd,whuwxl/etcd,sebrandon1/etcd,vimalk78/etcd,akauppi/etcd,rnaveiras/etcd,mqliang/etcd,mkumatag/etcd,cockroachdb/etcd,li-ang/etcd,ajityagaty/etcd,mkumatag/etcd,coreos/etcd,dannysauer/etcd,fanminshi/etcd,cockroachdb/etcd,gyuho/etcd,bluebreezecf/etcd,dannysauer/etcd,xiang90/etcd,sinsharat/etcd,mkumatag/etcd,joshix/etcd,mqliang/etcd,bluebreezecf/etcd,etcd-io/etcd,fanminshi/etcd,xiang90/etcd,etcd-io/etcd,glevand/coreos--etcd,rnaveiras/etcd,ajityagaty/etcd,sebrandon1/etcd,joshix/etcd,lorneli/etcd,coreos/etcd,li-ang/etcd,akauppi/etcd,bdarnell/etcd,heyitsanthony/etcd,lorneli/etcd,glevand/coreos--etcd,mqliang/etcd,heyitsanthony/etcd,sinsharat/etcd,vimalk78/etcd,ajityagaty/etcd,whuwxl/etcd,rnaveiras/etcd,bdarnell/etcd,etcd-io/etcd