[Nima] Set running flag to false in shutdownhook

[wojtask9]

Currently shutdownhook doesn't set flag running to false.
Setting to false is useful because we can test that WebServer.isRunning() in user custom hook

[spericas]

@wojtask9 Not sure I follow the use case you have in mind. Could you include a unit test? Also shutdown hooks are executed in an undefined order by the JDK.

[wojtask9]

@spericas
Not particular use case.
As you said shoutdown hooks are executed in an undefined order so there is no guarantee that this be useful but at least shows true about current webserver status.

Also this align with method stopIt() where such flag is correctly assigned

https://github.com/helidon-io/helidon/blob/main/nima/webserver/webserver/src/main/java/io/helidon/nima/webserver/LoomServer.java#L204