Method AddPriorityTask

src/common/thread_pool.h:89–94

    work_cv_.Signal();
  }
  void AddPriorityTask(const Task& task) {
    MutexLock lock(&mutex_);
    queue_.push_front(BGItem(0, get_micros(), task));
    ++pending_num_;
    work_cv_.Signal();
  }
  int64_t DelayTask(int64_t delay, const Task& task) {
    MutexLock lock(&mutex_);
    int64_t now_time = get_micros();