server: optimize message broadcast+targeted send

This commit slightly optimizes the process of broadcasting a message to
a list of peers, and also sending a set of messages to a target peer.

When broadcasting a message to a set of target peers, we now launch a
goroutine for each send as to not block the ChannelRouter on an
individual send. When sending a set of messages to a target peer, we
now give up the mutex as soon as we’ve access the map, rather than
holding onto it until the sending is complete.

rpcserver.go

@@ -1735,6 +1735,8 @@ func (r *rpcServer) GetNetworkInfo(context.Context, *lnrpc.NetworkInfoRequest) (
 		return nil, err
 	}
 
+	// TODO(roasbeef): graph diameter
+
 	// TODO(roasbeef): also add oldest channel?
 	//  * also add median channel size
 	return &lnrpc.NetworkInfo{