sortkey_count

统计hashkey下面的sortkey个数

    ///
    /// \brief sortkey_count
    ///     get sortkey count by hashkey from the cluster.
    /// \param hashkey
    /// used to decide which partition to get this k-v
    /// \param count
    /// the returned sortkey count
    /// \param timeout_milliseconds
    /// if wait longer than this value, will return time out error
    /// \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 sortkey_count(const std::string &hashkey,
                              int64_t &count,
                              int timeout_milliseconds = 5000,
                              internal_info *info = NULL) = 0;