EFA#93 EfaBoathouseBackgroundTask: Does not stop during shutdown

[cubitusgh2]

The efaBoathouseBackgroundtask can take quite some time for it's tasks. Especially checkBoatStatus can be time-consuming if there are a lot of Boats.

So when shutting down, the efaBackgroundTask may still be doing something in checkBoatStatus etc, but EfaBoathouseFrame.cancel may already be running and closing the project.

This leads to a lot of exceptions in debug mode, dependent on the timing of efaBoathouseBackgroundTask.

It would be more stable if the efaBoathouse BackgroundTask would be informed to shut down AND that efaBoathouseFrame.cancel would wait for some seconds for it to shut down.