* the application is complete. It does so by using daemon threads and adding a shutdown hook to
   * wait for their completion.
   *
   * <p>This is mainly for fixed thread pools. See {@link Executors#newScheduledThreadPool(int)}.
   *
   * @param executor the executor to modify to make sure it exits when the application is finished
   * @param terminationTimeout how long to wait for the executor to finish before terminating the