...

1. obitain a lock (write current version into 'xid' field)
2. add a row with new version.
3. delete aborted versions of row if exist (may be omitted for performance reasons)
4. update set xid_max of previous committed row if exist
5. delete previous committed versions (less or equal to cleanup version of tx snapshot) if existexist (may be omitted for performance reasons)

Cleanup of old versions