server: produces a channel backup upon shutdown

This is needed to keep channel.backup up-to-date if the node is stopped.

server.go

@@ -2504,6 +2504,23 @@ func (s *server) Stop() error {
 		if err := s.htlcNotifier.Stop(); err != nil {
 			srvrLog.Warnf("failed to stop htlcNotifier: %v", err)
 		}
+
+		// Update channel.backup file. Make sure to do it before
+		// stopping chanSubSwapper.
+		singles, err := chanbackup.FetchStaticChanBackups(
+			s.chanStateDB, s.addrSource,
+		)
+		if err != nil {
+			srvrLog.Warnf("failed to fetch channel states: %v",
+				err)
+		} else {
+			err := s.chanSubSwapper.ManualUpdate(singles)
+			if err != nil {
+				srvrLog.Warnf("Manual update of channel "+
+					"backup failed: %v", err)
+			}
+		}
+
 		if err := s.chanSubSwapper.Stop(); err != nil {
 			srvrLog.Warnf("failed to stop chanSubSwapper: %v", err)
 		}