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

Now we are using ContentReference to track contents in the content repository.
Now we are tracking references to overlays.
Trying to use server group definition for domain mode.
Simplifying ContentReference
WFCORE-211: Fixing overzealous cleaning of HC content.

Jira: https://issues.jboss.org/browse/WFCORE-213
and https://issues.jboss.org/browse/WFCORE-211