start(options = {}) {
    if (this._lastRequest) {
      if (!options.restart) {
        return Promise.resolve();
      }
      return this.stop({
        cancel: true,
        reason: 'Polling restart',
      }).then(() => {
        return this._polling();
      });
    }
    return this._polling();
  }
Start polling
@param  {Object} [options]
@param  {Object} [options.restart]
@return {Promise}