[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 removal logic shouldn't be in performRemove. It should be in a ResultHandler passed to completeStep, similar to what DeploymentRemoveHandler does. The ref should only be removed from the repo if the op has succeeded.

You won't be able to subclass AbstractRemoveStepHandler, but the only thing it does in this case is a call to context.removeResource(PathAddress.EMPTY_ADDRESS). Everything else gets bypassed in one way or another.