code: function synchronous(promiseFactory) {
    return function () {
      var promise = queue.then(promiseFactory);
      queue = promise.catch(noop); // squelch
      return promise;
    };
  }
docs: To mitigate 409s, we synchronize certain things that need to be
  done in a "transaction". So whatever promise is given to this
  function will execute sequentially.