discovery: introduce GossipSyncer signal delivery of chansSynced state

In this commit, we introduce another feature to the GossipSyncer in
which it can deliver a signal to an external caller once it reaches its
terminal chansSynced state. This is yet to be used, but will serve
useful with a round-robin sync mechanism, where we wait for to finish
syncing with a specific peer before moving on to the next.

discovery/syncer.go

@@ -271,6 +271,10 @@ type GossipSyncer struct {
 	// number of queries.
 	rateLimiter *rate.Limiter
 
+	// syncedSignal is a channel that, if set, will be closed when the
+	// GossipSyncer reaches its terminal chansSynced state.
+	syncedSignal chan struct{}
+
 	sync.Mutex
 
 	quit chan struct{}
@@ -470,6 +474,13 @@ func (g *GossipSyncer) channelGraphSyncer() {
 		// This is our final terminal state where we'll only reply to
 		// any further queries by the remote peer.
 		case chansSynced:
+			g.Lock()
+			if g.syncedSignal != nil {
+				close(g.syncedSignal)
+				g.syncedSignal = nil
+			}
+			g.Unlock()
+
 			// If we haven't yet sent out our update horizon, and
 			// we want to receive real-time channel updates, we'll
 			// do so now.
@@ -1049,6 +1060,24 @@ func (g *GossipSyncer) syncState() syncerState {
 	return syncerState(atomic.LoadUint32(&g.state))
 }
 
+// ResetSyncedSignal returns a channel that will be closed in order to serve as
+// a signal for when the GossipSyncer has reached its chansSynced state.
+func (g *GossipSyncer) ResetSyncedSignal() chan struct{} {
+	g.Lock()
+	defer g.Unlock()
+
+	syncedSignal := make(chan struct{})
+
+	syncState := syncerState(atomic.LoadUint32(&g.state))
+	if syncState == chansSynced {
+		close(syncedSignal)
+		return syncedSignal
+	}
+
+	g.syncedSignal = syncedSignal
+	return g.syncedSignal
+}
+
 // ProcessSyncTransition sends a request to the gossip syncer to transition its
 // sync type to a new one.
 //