function completeOutstandingRequest(fn) {
    try {
      fn.apply(null, sliceArgs(arguments, 1));
    } finally {
      outstandingRequestCount--;
      if (outstandingRequestCount === 0) {
        while (outstandingRequestCallbacks.length) {
          try {
            outstandingRequestCallbacks.pop()();
          } catch (e) {
            $log.error(e);
          }
        }
      }
    }
  }
Executes the `fn` function(supports currying) and decrements the `outstandingRequestCallbacks`
counter. If the counter reaches 0, all the `outstandingRequestCallbacks` are executed.