Anonymize checkin_staff and circ_staff columns when aging circulations

Current behavior:
1. Choose a patron in concerto that currently has items out
2. Log in to the OPAC and renew the item
3. In the staff client, Check in the item
4. In the database, DELETE the renewal circulation to age both the renewal and parent circulations.
5. SELECT circ_staff, checkin_staff FROM action.aged_circulation; -- note that the renewal circ's circ_staff and the parent circ's checkin_staff are both set to the patron's database ID.

Expected behavior: after a circulation is aged, the circ_staff and checkin_staff columns no longer refer to the patron, since the main goal of aging circulations is to make them more anonymous.

This bug is a spin-off from bug 2001728, where there is a lot of discussion that would be helpful in working this ticket.