discovery: always add chan announcements to the reject cache if err != ErrIgnored

In this commit, we make a change to always add chan announcements to the
reject cache if we didn't reject them for already existing.  Without
this change, if we end up rejecting a channel announcement say because
the channel is already spent or the funding transaction doesn't exist,
then we'll end up continually re-validating the same set of channels we
know will fail, when they're sent to us by peers.

Fixes #5191

discovery/gossiper.go

@@ -1741,6 +1741,10 @@ func (d *AuthenticatedGossiper) processNetworkAnnouncement(
 			} else {
 				log.Tracef("Router rejected channel "+
 					"edge: %v", err)
+
+				d.rejectMtx.Lock()
+				d.recentRejects[msg.ShortChannelID.ToUint64()] = struct{}{}
+				d.rejectMtx.Unlock()
 			}
 
 			nMsg.err <- err