[WFCORE-213] :  Clean unreferenced items from the content repository

We are using ContentReference to track contents in the content repository.
Se are tracking references to overlays.
Trying to use server group definition for domain mode.
Simplifying ContentReference
WFCORE-211: Fixing overzealous cleaning of HC content.
WFCORE-178: ContentRepositoryImpl should not call File.deleteOnExit

Jira: https://issues.jboss.org/browse/WFCORE-213
and https://issues.jboss.org/browse/WFCORE-211
and https://issues.jboss.org/browse/WFCORE-178
If you depend on an external service, you shouldn't shut it down, as you don't own it. If you do own the executor and shut it down, you should block until it's shut down.

One way or the other this should block until cleanup-in-progress completes (I haven't checked; maybe ContentRepositoryCleaner.stopScan() does.) Otherwise the ModelContoller service (ServerService or DomainModelControllerService) can shut down while a scan is still in progress.

If anything blocks, you need to use the StopContext.asynchronous() API.