refactor: create FetchLightningNode with no tx param

In preparation for adding a clean Graph DB interface, we create a
version of FetchLightningNode that doesnt allow a caller to provide in a
transaction.

server.go

@@ -4634,7 +4634,7 @@ func (s *server) fetchNodeAdvertisedAddrs(pub *btcec.PublicKey) ([]net.Addr, err
 		return nil, err
 	}
 
-	node, err := s.graphDB.FetchLightningNode(nil, vertex)
+	node, err := s.graphDB.FetchLightningNode(vertex)
 	if err != nil {
 		return nil, err
 	}