apply correct pre-fetch params to post updated rows

apply correct pre-fetch params to post updated rows

Fixed issue where using the ``post_update`` feature would apply incorrect
"pre-fetched" values to the ORM objects after a multi-row UPDATE process
completed.  These "pre-fetched" values would come from any column that had
an :paramref:`.Column.onupdate` callable or a version id generator used by
:paramref:`.orm.Mapper.version_id_generator`; for a version id generator
that delivered random identifiers like timestamps or UUIDs, this incorrect
data would lead to a DELETE statement against those same rows to fail in
the next step.

Fixes: #12748
Change-Id: Id12c7973f168604533762dfc01afbb9155b693a6