discovery: pass context to ProcessRemoteAnnouncement

With this, we move a context.TODO() out of the gossiper and into the
brontide package - this will be removed in a future PR which focuses on
threading contexts through that code.

discovery/syncer_test.go

@@ -454,7 +454,7 @@ func TestGossipSyncerApplyNoHistoricalGossipFilter(t *testing.T) {
 	}()
 
 	// We'll now attempt to apply the gossip filter for the remote peer.
-	syncer.ApplyGossipFilter(ctx, remoteHorizon)
+	require.NoError(t, syncer.ApplyGossipFilter(ctx, remoteHorizon))
 
 	// Ensure that the syncer's remote horizon was properly updated.
 	if !reflect.DeepEqual(syncer.remoteUpdateHorizon, remoteHorizon) {