[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
We'll either need to edit content.clean-obsolete-content.reply in LocalDescriptions.properties or change how this works. My text was wrong, as it's not the hashes of cleaned content that are in this list, it's the "content identifier".

Is returning the hashes of items that were actually deleted more useful? It's probably more work than it's worth, but returning both is also possible.