async_get_unordered_scanners

异步获取一个遍历所有数据的scanner

    ///
    /// \brief async get a bundle of scanners to iterate all k-v in table
    ///        scannners return by callback should be deleted when all scan complete
    /// \param max_split_count
    /// the number of scanners returned will always <= max_split_count
    /// \param options
    /// which used to indicate scan options, like timeout_milliseconds
    /// \param callback; return status and scanner in this callback
    ///
    virtual void
    async_get_unordered_scanners(int max_split_count,
                                 const scan_options &options,
                                 async_get_unordered_scanners_callback_t &&callback) = 0;