code: def on_finish(self):
          
          super(SessionRequestHandler, self).on_finish()
          LOGGER.debug('Entering SessionRequestHandler.on_finish: %s',
                       self.session.id)
          self.session.last_request_at = self.current_epoch()
          self.session.last_request_uri = self.request.uri
          if self.session.dirty:
              result = yield self.session.save()
              LOGGER.debug('on_finish yield save: %r', result)
          self.session = None
          LOGGER.debug('Exiting SessionRequestHandler.on_finish: %r',
                       self.session)
docs: Called by Tornado when the request is done. Update the session data
          and remove the session object.