shutdown method

Implementation

Future<void> shutdown() async {
  final done = _connections.map((connection) => connection.finish()).toList();
  if (_insecureServer != null) {
    done.add(_insecureServer!.close());
  }
  if (_secureServer != null) {
    done.add(_secureServer!.close());
  }
  await Future.wait(done);
  _insecureServer = null;
  _secureServer = null;
}