message: update task_id
diff: @@ -172,7 +172,7 @@ public:
  *_event = *ev;
  schedule();
  - _event->task_id = current_task_id++;
  + ev->task_id = _event->task_id = current_task_id++;
  queue.push(_event);
  _cv.notify_one();
  return true;
  