Method DelayTask

src/common/thread_pool.h:95–104

    work_cv_.Signal();
  }
  int64_t DelayTask(int64_t delay, const Task& task) {
    MutexLock lock(&mutex_);
    int64_t now_time = get_micros();
    int64_t exe_time = now_time + delay * 1000;
    BGItem bg_item(++last_task_id_, exe_time, task);
    time_queue_.push(bg_item);
    latest_[bg_item.id] = bg_item;
    work_cv_.Signal();
    return bg_item.id;
  }
  /// Cancel a delayed task
  /// if running, wait if non_block==false; return immediately if
  /// non_block==true