Method drain

src/common/WorkQueue.h:306–316

      ceph_assert(m_processing == 0);
    }
    void drain() {
      {
        // if this queue is empty and not processing, don't wait for other
        // queues to finish processing
        std::lock_guard l(m_pool->_lock);
        if (m_processing == 0 && m_items.empty()) {
          return;
        }
      }
      m_pool->drain(this);
    }
    void queue(T *item) {
      std::lock_guard l(m_pool->_lock);
      m_items.push_back(item);