clientv3: modify lock type.
clientv3: modify lock type.

mqliang/etcd,coreos/etcd,ajityagaty/etcd,mkumatag/etcd,gyuho/etcd,rnaveiras/etcd,mqliang/etcd,dannysauer/etcd,dannysauer/etcd,gyuho/etcd,rnaveiras/etcd,cockroachdb/etcd,mkumatag/etcd,ajityagaty/etcd,ajityagaty/etcd,etcd-io/etcd,li-ang/etcd,cockroachdb/etcd,rnaveiras/etcd,etcd-io/etcd,coreos/etcd,mkumatag/etcd,li-ang/etcd,mqliang/etcd,etcd-io/etcd