docs: public void addHandler(Handler handler, Stage stage)
    {
      if (!startStopLock.tryLock()) {
        throw new ISE("Cannot add a handler in the process of Lifecycle starting or stopping");
      }
      try {
        if (!state.get().equals(State.NOT_STARTED)) {
          throw new ISE("Cannot add a handler after the Lifecycle has started, it doesn't work that way.");
        }
        handlers.get(stage).add(handler);
      }
      finally {
        startStopLock.unlock();
      }
    }