channeldb: convert FetchOtherNode to ChannelGraph method

To prepare for the `kvdb.Backend` member of `ChannelEdgeInfo` being
removed, the `FetchOtherNode` method is moved from the `ChannelEdgeInfo`
to the `ChannelGraph` struct.

server.go

@@ -3107,7 +3107,9 @@ func (s *server) establishPersistentConnections() error {
 
 		// We'll now fetch the peer opposite from us within this
 		// channel so we can queue up a direct connection to them.
-		channelPeer, err := chanInfo.FetchOtherNode(tx, selfPub)
+		channelPeer, err := s.graphDB.FetchOtherNode(
+			tx, chanInfo, selfPub,
+		)
 		if err != nil {
 			return fmt.Errorf("unable to fetch channel peer for "+
 				"ChannelPoint(%v): %v", chanInfo.ChannelPoint,