code: @Override
    public Queue<NamespaceNotification> getClientNotificationQueue(long clientId) {
      ClientData clientData = clientsData.get(clientId);
      return (clientData == null) ? null : clientData.queue;
    }
docs: Gets the queue of notifications that should be sent to a client. It is
  important to send the notifications in this queue first before sending
  any other notification, or the order will be affected.
  
  @param clientId the id of the client for which we want to get the queued
  notifications.
  @return the queue with the notifications for this client. The returned
  queue is synchronized and no other synchronization mechanisms are
  required. null if the client is not registered.