little more work.  First, we need to delete all the entries for
oid+vid records in the currentVersions table, then if we delete all
thosey vid entries, garbage collect the version->vid and vid->version
tables.

pack(): Fix a slight misunderstanding of the semantics; if an object
isn't pointed to by root object (i.e. the magic \0\0\0\0\0\0\0\0 oid),
then it's current record doesn't keep it alive.