get_unordered_scanners

获取一个遍历所有数据的scanner

    ///
    /// \brief get a bundle of scanners to iterate all k-v in table
    ///        scanners should be deleted when 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 scanners
    /// out param, used to get k-v
    /// these pointers should be deleted
    /// \return
    /// int, the error indicates whether or not the operation is succeeded.
    /// this error can be converted to a string using get_error_string()
    ///
    virtual int get_unordered_scanners(int max_split_count,
                                       const scan_options &options,
                                       std::vector<pegasus_scanner *> &scanners) = 0;