Return all session unique ids recorded in prepare phase.

        :param ts: timestamp, default to current timestamp
        :return: set of session unique ids

def session_info(self, session):
        """Return all session unique ids recorded in prepare phase.

        :param ts: timestamp, default to current timestamp
        :return: set of session unique ids
        """
        _, sp_hkey = self._keygen(session)
        picked_event = self.r.hgetall(sp_hkey)
        event = {s(k): pickle.loads(v) for k, v in picked_event.items()}
        return event