Add a garbage collector for MFA challenges

Summary:
Depends on D19886. Ref T13222. Clean up MFA challenges after they expire.

(There's maybe some argument to keeping these around for a little while for debugging/forensics, but I suspect it would never actually be valuable and figure we can cross that bridge if we come to it.)

Test Plan:

• Ran bin/garbage collect --collector ... and saw old MFA challenges collected.
• Triggered a new challenge, GC'd again, saw it survive GC while still active.

Reviewers: amckinley

Reviewed By: amckinley

Maniphest Tasks: T13222

Differential Revision: https://secure.phabricator.com/D19888