Return all session unique ids recorded in prepare phase.

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

def prepare_info(self, ts=None):
        """Return all session unique ids recorded in prepare phase.

        :param ts: timestamp, default to current timestamp
        :return: set of session unique ids
        """
        sp_key = "%s:session_prepare" % self.namespace(ts or int(time.time()))
        return set(s(m) for m in self.r.smembers(sp_key))