Feature require: Safely clean up empty queue files

[hongweipeng]

Currently when the queue is empty, even after a long time, the file still takes up disk space. We need similar functionality：

func (d *diskQueue) CleanupIfQueueEmpty() error {
    d.Lock()
    defer d.Unlock()
    if d.Depth() ==  0 {
        return d.Empty()
    }
    return nil
}