docs: 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))