Instead of gc-ing previous revisions of nodes, record their address in the heap with a new slot in the base NODE class.

[kraison]

VG was intended to mimic couchdb's behavior of keeping old revisions of nodes around until manually garbage collected, or perhaps until some condition is triggered. It would be a simple matter to add an PREVIOUS-REVISION-POINTER field to the NODE base class and write the address of the old node to the heap with the new revision.