Load session from cookies.

async def load(self, request):
        """Load session from cookies."""
        if SESSION_KEY not in request:
            session = Session(self.cfg.secret, key=self.cfg.session_cookie,
                              max_age=self.cfg.max_age, domain=self.cfg.domain)
            session.load(request.cookies)
            self.app.logger.debug('Session loaded: %s', session)
            request[SESSION_KEY] = request.session = session
        return request[SESSION_KEY]