discovery: signal allow for ignored channel announcements

This commit makes the `handleChanAnnouncement` always returning `true`
for messages processed but ignored by router, even when the extracted
announcements are empty.
Previously we'd return false when the announcements are empty, which
could cause `ChannelUpdate`s being ignored since the validation barrier
will signal deny for the jobs. This can easily be trigger using
following setup,
1. Alice connects to Bob and open a channel.
2. Alice connects to Carol, Bob connects to Carol.
3. Once the channel is open, Alice and Bob will both announce it to Carol.

At some point, we'd have the following messages in Carol's node,
- Alice's ChannelAnnouncement
- Alice's ChannelUpdates, for both directions
- Bob's ChannelAnnouncement
- Bob's ChannelUpdates, for both directions

And a bug could happen, if,
- Alice's ChannelAnnouncement is processed by router, hence added to db,
  but not reporting back to gossiper yet, so the validation barrier
  hasn't sent signal allow.
- Bob's ChannelAnnouncement is processed by router, and returned
  `ErrIgnored` as the edge info is already in db, and reported back to
  gossiper, the validation barrier will signal deny to all the
  ChannelUpdates jobs.
- Depending on how fast Alice's ChannelAnnouncement is processed, we may
  get zero to four denies to the above ChannelUpdates, causing a channel
  edge policy never being updated.

discovery/gossiper.go

@@ -2372,11 +2372,12 @@ func (d *AuthenticatedGossiper) handleChanAnnouncement(nMsg *networkMsg,
 			// If while processing this rejected edge, we realized
 			// there's a set of announcements we could extract,
 			// then we'll return those directly.
-			if len(anns) != 0 {
-				nMsg.err <- nil
-				return anns, true
-			}
+			//
+			// NOTE: since this is an ErrIgnored, we can return
+			// true here to signal "allow" to its dependants.
+			nMsg.err <- nil
 
+			return anns, true
 		} else {
 			// Otherwise, this is just a regular rejected edge.
 			key := newRejectCacheKey(