discovery: interpret block range from ReplyChannelRange messages

We move from our legacy way of interpreting ReplyChannelRange messages
which was incorrect. Previously, we'd rely on the Complete field of the
ReplyChannelRange message to determine when our peer had sent all of
their replies. Now, we properly adhere to the specification by
interpreting the block ranges of these messages as intended.

Due to the large number of nodes deployed with the previous method, we
still maintain and detect when we are communicating with them, such that
we are still able to sync with them for backwards compatibility.

discovery/sync_manager_test.go

@@ -529,12 +529,16 @@ func assertSyncerStatus(t *testing.T, s *GossipSyncer, syncState syncerState,
 func assertTransitionToChansSynced(t *testing.T, s *GossipSyncer, peer *mockPeer) {
 	t.Helper()
 
-	assertMsgSent(t, peer, &lnwire.QueryChannelRange{
+	query := &lnwire.QueryChannelRange{
 		FirstBlockHeight: 0,
 		NumBlocks:        math.MaxUint32,
-	})
+	}
+	assertMsgSent(t, peer, query)
 
-	s.ProcessQueryMsg(&lnwire.ReplyChannelRange{Complete: 1}, nil)
+	s.ProcessQueryMsg(&lnwire.ReplyChannelRange{
+		QueryChannelRange: *query,
+		Complete:          1,
+	}, nil)
 
 	chanSeries := s.cfg.channelSeries.(*mockChannelGraphTimeSeries)