docs: def authenticatedUserForKey(self, key):
          """
          Find a persistent session for a user.
  
          @type key: L{bytes}
          @param key: The persistent session identifier.
  
          @rtype: L{bytes} or C{None}
          @return: The avatar ID the session belongs to, or C{None} if no such
              session exists.
          """
          session = self.store.findFirst(
              PersistentSession, PersistentSession.sessionKey == key)
          if session is None:
              return None
          else:
              session.renew()
              return session.authenticatedAs