avoid race conditions for OnPeerUpdate (#3525)

https://github.com/seaweedfs/seaweedfs/issues/3524

weed/server/master_server.go

@@ -116,7 +116,7 @@ func NewMasterServer(r *mux.Router, option *MasterOption, peers map[string]pb.Se
 	}
 	ms.boundedLeaderChan = make(chan int, 16)
 
-	ms.MasterClient.OnPeerUpdate = ms.OnPeerUpdate
+	ms.MasterClient.SetOnPeerUpdateFn(ms.OnPeerUpdate)
 
 	seq := ms.createSequencer(option)
 	if nil == seq {