Method delay

src/task_pool.h:158–186

     */
    template<class X, class Y>
    void delay(task_id_t task_id, std::chrono::duration<X, Y> duration) {
      std::lock_guard<std::mutex> lg(_task_mutex);

      auto it = _timer_tasks.begin();
      for (; it < _timer_tasks.cend(); ++it) {
        const __task &task = std::get<1>(*it);

        if (&*task == task_id) {
          std::get<0>(*it) = std::chrono::steady_clock::now() + duration;

          break;
        }
      }

      if (it == _timer_tasks.cend()) {
        return;
      }

      // smaller time goes to the back
      auto prev = it - 1;
      while (it > _timer_tasks.cbegin()) {
        if (std::get<0>(*it) > std::get<0>(*prev)) {
          std::swap(*it, *prev);
        }

        --prev;
        --it;
      }
    }

    bool cancel(task_id_t task_id) {
      std::lock_guard lg(_task_mutex);