v3rpc: do not send closing event

When a watch stream closes, both of the watcher.Chan and closec
will be closed.

If watcher.Chan is closed, we should not send out the empty event.
Sending the empty is wrong and waste a lot of CPU resources.
Instead we should just return.

etcdserver/api/v3rpc/watch.go

@@ -61,7 +61,10 @@ func (ws *watchServer) Watch(stream pb.Watch_WatchServer) error {
 func sendLoop(stream pb.Watch_WatchServer, watcher storage.Watcher, closec chan struct{}) {
 	for {
 		select {
-		case e := <-watcher.Chan():
+		case e, ok := <-watcher.Chan():
+			if !ok {
+				return
+			}
 			err := stream.Send(&pb.WatchResponse{Event: &e})
 			if err != nil {
 				return