discovery: ensure we only broadcast NodeAnnouncements of public nodes

In this commit, we modify the gossiper to no longer broadcast
NodeAnnouncements of nodes who intend to remain private. We do this to
prevent leaking their information to the greater network.

discovery/gossiper.go

@@ -1613,13 +1613,26 @@ func (d *AuthenticatedGossiper) processNetworkAnnouncement(
 			return nil
 		}
 
-		// Node announcement was successfully proceeded and know it
-		// might be broadcast to other connected nodes.
-		announcements = append(announcements, networkMsg{
-			peer:   nMsg.peer,
-			source: nMsg.source,
-			msg:    msg,
-		})
+		// In order to ensure we don't leak unadvertised nodes, we'll
+		// make a quick check to ensure this node intends to publicly
+		// advertise itself to the network.
+		isPublic, err := d.cfg.Router.IsPublicNode(node.PubKeyBytes)
+		if err != nil {
+			log.Errorf("Unable to determine if node %x is "+
+				"advertised: %v", node.PubKeyBytes, err)
+			nMsg.err <- err
+			return nil
+		}
+
+		// If it does, we'll add their announcement to our batch so that
+		// it can be broadcast to the rest of our peers.
+		if isPublic {
+			announcements = append(announcements, networkMsg{
+				peer:   nMsg.peer,
+				source: nMsg.source,
+				msg:    msg,
+			})
+		}
 
 		nMsg.err <- nil
 		// TODO(roasbeef): get rid of the above