multi_set

写多条数据（同一hashkey下面）

    ///
    /// \brief multi_set (guarantee atomicity)
    ///     store multiple k-v of the same hashkey to the cluster.
    /// \param hashkey
    /// used to decide which partition to put this k-v
    /// \param kvs
    /// all <sortkey,value> pairs to be set. should not be empty
    /// \param timeout_milliseconds
    /// if wait longer than this value, will return time out error
    /// \param ttl_seconds
    /// time to live of this value, if expired, will return not found; 0 means no ttl
    /// \return
    /// int, the error indicates whether or not the operation is succeeded.
    /// this error can be converted to a string using get_error_string().
    /// return PERR_INVALID_ARGUMENT if param kvs is empty.
    ///
    virtual int multi_set(const std::string &hashkey,
                          const std::map<std::string, std::string> &kvs,
                          int timeout_milliseconds = 5000,
                          int ttl_seconds = 0,
                          internal_info *info = NULL) = 0;