improve priority_queue to prevent starvation

[qinzuoyan]

now the priority queue's dequeue() is:

    T dequeue_impl(/*out*/ long &ct, bool pop = true)
    {
        if (_count == 0) {
            ct = 0;
            return nullptr;
        }

        ct = --_count;

        int index = priority_count - 1;
        for (; index >= 0; index--) {
            if (_items[index].size() > 0) {
                break;
            }
        }

        assert(index >= 0); // "must find something");
        auto c = _items[index].front();
        _items[index].pop();
        return c;
    }

if the HIGH priority queue is always not empty, the task in COMMON/LOW queue may be starved.

we can refer to the implementation of nfs_client_impl.