docs: def set_bytes_transferred(self, bytes_transferred):
          ''' set the number of bytes transferred - if it has changed return True '''
          _changed = False
          if bytes_transferred:
              _changed = (self._bytes_transferred != int(bytes_transferred))
              if _changed:
                  self._bytes_transferred = int(bytes_transferred)
                  logger.debug("(%s) BytesTransferred: %d" % (
                               self.session_id, self._bytes_transferred))
              if AsperaSession.PROGRESS_MSGS_SEND_ALL:
                  return True
          return _changed