master and volume server: avoid race condition

The volume server may disconnect and reconnect to the same master.

The master's unregistration may happen after the reconnection. Thus the volume server will disappear.

weed/server/volume_grpc_client_to_master.go

@@ -35,6 +35,9 @@ func (vs *VolumeServer) heartbeat() {
 	for {
 		for _, master := range vs.SeedMasterNodes {
 			if newLeader != "" {
+				// the new leader may actually is the same master
+				// need to wait a bit before adding itself
+				time.Sleep(3 * time.Second)
 				master = newLeader
 			}
 			masterGrpcAddress, parseErr := pb.ParseServerToGrpcAddress(master)