Called by Tornado when the request is done. Update the session data
        and remove the session object.

def on_finish(self):
        """Called by Tornado when the request is done. Update the session data
        and remove the session object.

        """
        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)