Lua队列收缩：清空重建的内存回收

-- 错误：出队一步清一步，处处是清理开销
function pop()
    local v = data[qHead]
    data[qHead] = nil
    data[qHead + 1] = nil
    qHead = qHead + 1
    return v
end

local function pop()
    if q._index >= q._border then
        q._data = {}
        q._index, q._border = 1, 1
        return nil
    end
    local v = q._data[q._index]
    q._data[q._index] = nil
    q._index = q._index + 1
    return v
end