channeldb/graph: filter zombie channels in FilterKnownChanIDs

This commit modifies FilterKnownChanIDs to skip edges that
we ourselves have deemed zombies. This prevents us from requesting
the updates from them, as this wastes bandwidth and cpu cycles.

channeldb/graph_test.go

@@ -1741,6 +1741,24 @@ func TestFilterKnownChanIDs(t *testing.T) {
 		chanIDs = append(chanIDs, chanID.ToUint64())
 	}
 
+	const numZombies = 5
+	zombieIDs := make([]uint64, 0, numZombies)
+	for i := 0; i < numZombies; i++ {
+		channel, chanID := createEdge(
+			uint32(i*10+1), 0, 0, 0, node1, node2,
+		)
+		if err := graph.AddChannelEdge(&channel); err != nil {
+			t.Fatalf("unable to create channel edge: %v", err)
+		}
+		if err := graph.MarkEdgeZombie(
+			chanID.ToUint64(), node1.PubKeyBytes, node2.PubKeyBytes,
+		); err != nil {
+			t.Fatalf("unable to mark edge zombie: %v", err)
+		}
+
+		zombieIDs = append(zombieIDs, chanID.ToUint64())
+	}
+
 	queryCases := []struct {
 		queryIDs []uint64
 
@@ -1751,6 +1769,11 @@ func TestFilterKnownChanIDs(t *testing.T) {
 		{
 			queryIDs: chanIDs,
 		},
+		// If we attempt to filter out all zombies that we know of, the
+		// response should be the empty set.
+		{
+			queryIDs: zombieIDs,
+		},
 
 		// If we query for a set of ID's that we didn't insert, we
 		// should get the same set back.