multi_get_sortkeys

获取hashkey下面的多个sortkey，不返回value

    ///
    /// \brief multi_get_sortkeys
    ///     get multiple sort keys by hash key from the cluster.
    ///     only fetch sort keys, but not fetch values.
    /// \param hashkey
    /// used to decide which partition to get this k-v
    /// \param sortkeys
    /// the returned sort keys will be put into it.
    /// \param max_fetch_count
    /// max count of sort keys to be fetched. max_fetch_count <= 0 means no limit.
    /// \param max_fetch_size
    /// max size of sort keys to be fetched. max_fetch_size <= 0 means no limit.
    /// \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().
    /// returns PERR_OK if fetch done, even no data is returned.
    /// returns PERR_INCOMPLETE is only partial data is fetched.
    ///
    virtual int multi_get_sortkeys(const std::string &hashkey,
                                   std::set<std::string> &sortkeys,
                                   int max_fetch_count = 100,
                                   int max_fetch_size = 1000000,
                                   int timeout_milliseconds = 5000,
                                   internal_info *info = NULL) = 0;

注：max_fetch_count 和 max_fetch_size 分别限制返回的sortkey的个数与总大小（计算大小的时候，为每条sortkey都计算一次hashkey的大小）