DefaultServerFactoryTest#testGracefulShutdown works as expected

After upgrading to Jersey 2.11 in #706  and to Jetty 9.2.3 in #453 this test works as expected. 
I can't reproduce the issue.

````java
    @Test
    public void testManyGracefulShutdowns() {
        int successes = 0;
        int fails = 0;
        for (int i = 0; i < 1000; i++) {
            try {
                testGracefulShutdown();
                successes++;
            } catch (Exception e) {
                e.printStackTrace();
                fails++;
            }
        }
        System.out.printf("%d successes. %d fails", successes, fails);
    }
````

````
1000 successes. 0 fails
````

As I beleive, Jersey 1.* is to blame, because I didn't reproduce this issue with Jersey 2.11 and Jetty 9.0.7