f = function (worker) {
    DEBUG_PORT_ALLOCATOR.releasePort(worker.debugPort);
    this._removeWorkerFromList(worker);
  }
Close all active workers. Tasks currently being executed will be finished first.
@param {boolean} [force=false]   If false (default), the workers are terminated
                                 after finishing all tasks currently in
                                 progress. If true, the workers will be
                                 terminated immediately.
@param {number} [timeout]        If provided and non-zero, worker termination promise will be rejected
                                 after timeout if worker process has not been terminated.
@return {Promise.<void, Error>}