half-sync-half-async pattern need to enhance

[npczwh]

In the App class of half-sync-half-async pattern, the workers in threadpool is still running when the main thread has finished. This also makes the AppTest less effective (after the test, we expect to see task results, but we get nothing because junit will terminate jvm when the main thread has finished).
To improve on it, we can add a close method to the AsynchronousService class and call the method at the end of the main thread. I can create a PR, if needed.