start method
Starts this server.
Implementation
Future<bool> start() async {
  if (_started) return true;
  _started = true;
  _starting = true;
  return _startAsync = _startImpl().then(
    (ok) {
      _starting = false;
      _startAsync = null;
      return ok;
    },
    onError: (e, s) {
      _starting = false;
      _startAsync = null;
      _log.severe("Initialization error", e, s);
      throw e;
    },
  );
}