remove buffering from the callback IO object implementation

remove buffering from the callback IO object implementation

   - the callback IO object did its own buffering, controlled by the
     maxbuffer parameter supplied to the read() and write() methods.
     This buffering has been removed, to avoid redundancy with the
     common io_glue buffering.

   - the callback IO object new tests the result of calling the close
     callback, which should return true for success.