It is well known that OST defines garbage collection treshold for every record in database.
By its nature, GTT's data is local to one attachment. Again, by its nature, OST is global state.
While (global) OST could be stuck or slowly advanced, there is no need to look at it to detect
if given record verion in GTT is mature for garbage collection. Instead, attachment-local
counter could be maintained to define garbage collection treshold for GTT's records in given
attachement.