Add open_data_buffer()/close_data_buffer() (was get_data_buffer()) methods

Currently (v24) we have the following call to pass the data to provider:

wsrep_status_t (*append_data)(wsrep_t* wsrep,
                                  wsrep_ws_handle_t* ws_handle,
                                  const struct wsrep_buf* data,
                                  size_t count,
                                  enum wsrep_data_type type,
                                  wsrep_bool_t copy);

This assumes that the caller already has the data in a memory buffer. However this is not always possible (or convenient, e.g. interface to data is read()/write() via file descriptor). To cover such cases and minimize the need for memory copying, provide the following call:

wsrep_status_t (*get_data_buffer)(wsrep_t* wsrep,
                                  wsrep_ws_handle_t* ws_handle,
                                  size_t buf_size,
                                  enum wsrep_data_type type,
                                  void** buf);

Which would provide the application with a buffer to copy data to.