discovery: implement ChannelAnnouncement banning

This commit hooks up the banman to the gossiper:
- peers that are banned and don't have a channel with us will get
  disconnected until they are unbanned.
- peers that are banned and have a channel with us won't get
  disconnected, but we will ignore their channel announcements until
  they are no longer banned. Note that this only disables gossip of
  announcements to us and still allows us to open channels to them.

discovery/sync_manager.go

@@ -22,6 +22,9 @@ const (
 	// force a historical sync to ensure we have as much of the public
 	// network as possible.
 	DefaultHistoricalSyncInterval = time.Hour
+
+	// filterSemaSize is the capacity of gossipFilterSema.
+	filterSemaSize = 5
 )
 
 var (
@@ -161,12 +164,22 @@ type SyncManager struct {
 	// duration of the connection.
 	pinnedActiveSyncers map[route.Vertex]*GossipSyncer
 
+	// gossipFilterSema contains semaphores for the gossip timestamp
+	// queries.
+	gossipFilterSema chan struct{}
+
 	wg   sync.WaitGroup
 	quit chan struct{}
 }
 
 // newSyncManager constructs a new SyncManager backed by the given config.
 func newSyncManager(cfg *SyncManagerCfg) *SyncManager {
+
+	filterSema := make(chan struct{}, filterSemaSize)
+	for i := 0; i < filterSemaSize; i++ {
+		filterSema <- struct{}{}
+	}
+
 	return &SyncManager{
 		cfg:          *cfg,
 		newSyncers:   make(chan *newSyncer),
@@ -178,7 +191,8 @@ func newSyncManager(cfg *SyncManagerCfg) *SyncManager {
 		pinnedActiveSyncers: make(
 			map[route.Vertex]*GossipSyncer, len(cfg.PinnedSyncers),
 		),
-		quit: make(chan struct{}),
+		gossipFilterSema: filterSema,
+		quit:             make(chan struct{}),
 	}
 }
 
@@ -507,7 +521,7 @@ func (m *SyncManager) createGossipSyncer(peer lnpeer.Peer) *GossipSyncer {
 		maxQueryChanRangeReplies:  maxQueryChanRangeReplies,
 		noTimestampQueryOption:    m.cfg.NoTimestampQueries,
 		isStillZombieChannel:      m.cfg.IsStillZombieChannel,
-	})
+	}, m.gossipFilterSema)
 
 	// Gossip syncers are initialized by default in a PassiveSync type
 	// and chansSynced state so that they can reply to any peer queries or