discovery: attempt to request the full chan ann for stray chan updates

In this commit, we extend the AuthenticatedGossiper to take advantage of
the new query features in the case that it gets a channel update w/o
first receiving the full channel announcement. If this happens, we'll
attempt to find a syncer that's fully synced, and request the channel
announcement from it.

discovery/syncer.go

@@ -256,7 +256,8 @@ func (g *gossipSyncer) channelGraphSyncer() {
 
 	for {
 		state := atomic.LoadUint32(&g.state)
-		log.Debugf("gossipSyncer(%x): state=%v", g.peerPub[:], state)
+		log.Debugf("gossipSyncer(%x): state=%v", g.peerPub[:],
+			syncerState(state))
 
 		switch syncerState(state) {
 		// When we're in this state, we're trying to synchronize our