routing+discovery: extend ChannelGraphSource with zombie index methods

discovery/gossiper.go

@@ -1793,7 +1793,6 @@ func (d *AuthenticatedGossiper) processNetworkAnnouncement(
 		if d.cfg.Router.IsStaleEdgePolicy(
 			msg.ShortChannelID, timestamp, msg.ChannelFlags,
 		) {
-
 			nMsg.err <- nil
 			return nil
 		}