get_scanner

获取针对某个hashkey下的sortkey区间 [sortkeyA ~ sortkeyB)的一个scanner

    ///
    /// \brief get hash scanner
    ///     get scanner for [start_sortkey, stop_sortkey) of hashkey
    /// \param hashkey
    /// cannot be empty
    /// \param start_sortkey
    /// sortkey to start with
    /// \param stop_sortkey
    /// sortkey to stop. ""(empty string) represents the max key
    /// \param options
    /// which used to indicate scan options, like which bound is inclusive
    /// \param scanner
    /// out param, used to get k-v
    /// this pointer should be deleted when scan complete
    /// \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_scanner(const std::string &hashkey,
                            const std::string &start_sortkey, // start from beginning if this set ""
                            const std::string &stop_sortkey,  // to the last item if this set ""
                            const scan_options &options,
                            pegasus_scanner *&scanner) = 0;