discovery: skip sending channel_announcement with no channel_updates

Base on BOLT07:
> If a channel_announcement has no corresponding channel_updates:
>   - MUST NOT send the channel_announcement.

discovery/chan_series.go

@@ -136,7 +136,21 @@ func (c *ChanSeries) UpdatesInHorizon(chain chainhash.Hash,
 			return nil, err
 		}
 
-		updates = append(updates, chanAnn)
+		// Create a slice to hold the `channel_announcement` and
+		// potentially two `channel_update` msgs.
+		//
+		// NOTE: Based on BOLT7, if a channel_announcement has no
+		// corresponding channel_updates, we must not send the
+		// channel_announcement. Thus we use this slice to decide we
+		// want to send this `channel_announcement` or not. By the end
+		// of the operation, if the len of the slice is 1, we will not
+		// send the `channel_announcement`. Otherwise, when sending the
+		// msgs, the `channel_announcement` must be sent prior to any
+		// corresponding `channel_update` or `node_annoucement`, that's
+		// why we create a slice here to maintain the order.
+		chanUpdates := make([]lnwire.Message, 0, 3)
+		chanUpdates = append(chanUpdates, chanAnn)
+
 		if edge1 != nil {
 			// We don't want to send channel updates that don't
 			// conform to the spec (anymore).
@@ -145,18 +159,28 @@ func (c *ChanSeries) UpdatesInHorizon(chain chainhash.Hash,
 				log.Errorf("not sending invalid channel "+
 					"update %v: %v", edge1, err)
 			} else {
-				updates = append(updates, edge1)
+				chanUpdates = append(chanUpdates, edge1)
 			}
 		}
+
 		if edge2 != nil {
 			err := netann.ValidateChannelUpdateFields(0, edge2)
 			if err != nil {
 				log.Errorf("not sending invalid channel "+
 					"update %v: %v", edge2, err)
 			} else {
-				updates = append(updates, edge2)
+				chanUpdates = append(chanUpdates, edge2)
 			}
 		}
+
+		// If there's no corresponding `channel_update` to send, skip
+		// sending this `channel_announcement`.
+		if len(chanUpdates) < 2 {
+			continue
+		}
+
+		// Append the all the msgs to the slice.
+		updates = append(updates, chanUpdates...)
 	}
 
 	// Next, we'll send out all the node announcements that have an update