server: make sendToPeer async

This commit removes another case of unnecessary blockage, by modifying
the sendToPeer method to be fully asynchronous. From the PoV of the
callers that utilize this method currently, there’s no reason to block
until the completion of this method. Additionally, as the graph grows
larger without more intelligent the number of messages sent during
initial dump will start to be prohibitive to waiting for full
completion before proceeding.

server.go

@@ -908,7 +908,7 @@ func (s *server) sendToPeer(target *btcec.PublicKey, msgs []lnwire.Message) erro
 		return err
 	}
 
-	s.sendPeerMessages(targetPeer, msgs)
+	go s.sendPeerMessages(targetPeer, msgs)
 
 	return nil
 }