Remove orphaned nodes/documents

As part of DetailedGC (also see OAK-10199) we also need to clean up documents that (for some reason) have become orphaned. Orphaned nodes are nodes without a parent, i.e. they fulfill two criterias:

• they cannot be traversed to - the traversed state would be null / non-existant
• but reading the node via getNodeAtRevision would properly resolve in an existing node