Issue #4919 - WebSocket container graceful stop

[lachlan-roberts]

Closes #4919

Make sure all active websocket connections are closed before the connections are hard closed.

Server.doStop() now initiates a graceful shutdown but only waits for the CompletableFuture if the stopTimeout is set.

[lachlan-roberts]

@gregw implemented the changes.

There is a problem that there is no API for a Javax WebSocketContainer stop timeout, so it can't be stopped gracefully. I could try putting a short set length stop timeout to give some minimal effort to close gracefully, but not sure how that would interact with the ServerContainer shutdown.

[gregw]

I'm approving this other than a javadoc and naming niggle and a suggestion to make cancel work a bit better.

I think this PR should be merged, but then we should start another one looking at other improvements in graceful (eg like actually calling cancel if the stopTimeout expires).

[lachlan-roberts]

@gregw I didn't like the state of this PR so I've done some more changes and now I think it is more in line with the way Graceful should be used.

Now the SessionTracker always remains a bean on the websocket container and the SessionTracker is the one to implement Graceful not the container itself. The SessionTracker is then shutdown by Graceful.shutdown(Container) which will find the SessionTracker in the Container hierarchy.

[chylek-qr]

EDIT: Moved to #10918

[joakime]

@chylek-qr please open a new issue, a comment on a 3 year old PR is unlikely to be tracked.