async_get

异步读取一条数据

    ///
    /// \brief asynchronous get
    ///     get value by key from the cluster.
    ///     will not be blocked, return immediately.
    /// \param hashkey
    /// used to decide which partition to get this k-v
    /// \param sortkey
    /// all the k-v under hashkey will be sorted by sortkey.
    /// \param callback
    /// the callback function will be invoked after operation finished or error occurred.
    /// \param timeout_milliseconds
    /// if wait longer than this value, will return time out error
    /// \return
    /// void.
    ///
    virtual void async_get(const std::string &hashkey,
                           const std::string &sortkey,
                           async_get_callback_t &&callback = nullptr,
                           int timeout_milliseconds = 5000) = 0;