GracefulShutdownHandler has race condition that results in shutdownComplete not called

UNDERTOW-1099 fix introduced a different bug which results in listeners not being called.

    private void decrementRequests() {
        if (shutdown) {
            //we don't read the request count until after checking the shutdown variable
            //otherwise we could read the request count as zero, a new request could state, and then we shutdown
            //see https://issues.jboss.org/browse/UNDERTOW-1099
            long active = activeRequestsUpdater.decrementAndGet(this);
            synchronized (lock) {
                if (active == 0) {
                    shutdownComplete();
                }
            }
        } else {
            activeRequestsUpdater.decrementAndGet(this);
        }
    }

Thread T1 calls decrementRequests. Shutdown is false, so this thread will follow the 'else' path in preparation for decrementAndGet
Thread T2 calls shutdown. Increments from 1 to 2, sets shutdown=true, and calls decrementRequests where decrementAndGet results in 1
Thread T1 completes the decrementRequests invocation reducing the active request count to zero in the else path without calling shutdownComplete.

flaviarnn I don't have permissions to reopen this, would you prefer that I open a new ticket?

I would propose using a bit mask to represent both shutdownRequested and shutdownComplete to allow state changes within individual cas operations.