etcdserver: do not send out out of date appResp

etcdserver/server.go

@@ -938,11 +938,20 @@ func (s *EtcdServer) publish(timeout time.Duration) {
 
 // TODO: move this function into raft.go
 func (s *EtcdServer) send(ms []raftpb.Message) {
-	for i := range ms {
+	sentAppResp := false
+	for i := len(ms) - 1; i >= 0; i-- {
 		if s.cluster.IsIDRemoved(types.ID(ms[i].To)) {
 			ms[i].To = 0
 		}
 
+		if ms[i].Type == raftpb.MsgAppResp {
+			if sentAppResp {
+				ms[i].To = 0
+			} else {
+				sentAppResp = true
+			}
+		}
+
 		if ms[i].Type == raftpb.MsgSnap {
 			// There are two separate data store: the store for v2, and the KV for v3.
 			// The msgSnap only contains the most recent snapshot of store without KV.