discovery: thread contexts to syncer

The `GossiperSyncer` makes various calls to the `ChannelGraphTimeSeries`
interface which threads through to the graph DB. So in preparation for
threading context through to all the methods on that interface, we
update the GossipSyncer accordingly by passing contexts through.

Two `context.TODO()`s are added in this commit. They will be removed in
the upcoming commits.

discovery/gossiper.go

@@ -860,6 +860,8 @@ func (d *AuthenticatedGossiper) stop() {
 func (d *AuthenticatedGossiper) ProcessRemoteAnnouncement(msg lnwire.Message,
 	peer lnpeer.Peer) chan error {
 
+	ctx := context.TODO()
+
 	log.Debugf("Processing remote msg %T from peer=%x", msg, peer.PubKey())
 
 	errChan := make(chan error, 1)
@@ -907,7 +909,7 @@ func (d *AuthenticatedGossiper) ProcessRemoteAnnouncement(msg lnwire.Message,
 
 		// If we've found the message target, then we'll dispatch the
 		// message directly to it.
-		if err := syncer.ApplyGossipFilter(m); err != nil {
+		if err := syncer.ApplyGossipFilter(ctx, m); err != nil {
 			log.Warnf("Unable to apply gossip filter for peer=%x: "+
 				"%v", peer.PubKey(), err)
 
@@ -1404,7 +1406,7 @@ func (d *AuthenticatedGossiper) sendLocalBatch(annBatch []msgWithSenders) {
 
 // sendRemoteBatch broadcasts a list of remotely generated announcements to our
 // peers.
-func (d *AuthenticatedGossiper) sendRemoteBatch(_ context.Context,
+func (d *AuthenticatedGossiper) sendRemoteBatch(ctx context.Context,
 	annBatch []msgWithSenders) {
 
 	syncerPeers := d.syncMgr.GossipSyncers()
@@ -1413,7 +1415,7 @@ func (d *AuthenticatedGossiper) sendRemoteBatch(_ context.Context,
 	// that have active gossip syncers active.
 	for pub, syncer := range syncerPeers {
 		log.Tracef("Sending messages batch to GossipSyncer(%s)", pub)
-		syncer.FilterGossipMsgs(annBatch...)
+		syncer.FilterGossipMsgs(ctx, annBatch...)
 	}
 
 	for _, msgChunk := range annBatch {