[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
This should be created in start. A service should be able to go through start/stop/start/stop many times without failing, and this one can't because the executor is shut down in stop.

Probably simpler is to inject ServerService.JBOSS_SERVER_SCHEDULED_EXECUTOR (on a server) or HostControllerService.HC_SCHEDULED_EXECUTOR_SERVICE_NAME on an HC. That will simplify stop by removing the need asynchronous shutdown I'm about to mention.