multi+refactor: create ForEachNodeChannel with no tx param

In prep for a clean Graph DB interface, we add a version of
ForEachNodeChannel that does not take in an existing db transaction.

server.go

@@ -3119,7 +3119,7 @@ func (s *server) establishPersistentConnections() error {
 	// TODO(roasbeef): instead iterate over link nodes and query graph for
 	// each of the nodes.
 	selfPub := s.identityECDH.PubKey().SerializeCompressed()
-	err = s.graphDB.ForEachNodeChannel(nil, sourceNode.PubKeyBytes, func(
+	err = s.graphDB.ForEachNodeChannel(sourceNode.PubKeyBytes, func(
 		tx kvdb.RTx,
 		chanInfo *models.ChannelEdgeInfo,
 		policy, _ *models.ChannelEdgePolicy) error {