When using GRPC, it's possible that we never connect to another server

[felipejfc]

https://github.com/topfreegames/pitaya/blob/master/cluster/grpc_rpc_client.go#L234-L238

	conn, err := grpc.DialContext(ctxT, address, grpc.WithInsecure())
	if err != nil {
		logger.Log.Errorf("unable to connect to server %s at %s: %v", sv.ID, address, err)
		return
	}

This piece of code may lead us to having a known server that we never connect to, maybe we should remove the return and leave GRPC trying to reconnect and if the server is really messed up, eventually it will be removed and deleted from the local sd storage