async_get_scanner

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

    ///
    /// \brief async get hash scanner
    ///     get scanner for [start_sortkey, stop_sortkey) of hashkey
    ///     will not be blocked, return immediately.
    /// \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 callback
    /// return status and scanner in callback, and the latter should be deleted when scan complete
    ///
    virtual void
    async_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,
                      async_get_scanner_callback_t &&callback) = 0;