Hold a lock while collecting garbage

Summary:
Fixes T11771. Adds a lock around each GC process so we don't try to, e.g., delete old files on two machines at once just because they're both running trigger daemons.

The other aspects of this daemon (actual triggers; nuance importers) already have separate locks.

Test Plan: Ran bin/phd debug trigger --trace, saw daemon acquire locks and collect garbage.

Reviewers: chad

Reviewed By: chad

Maniphest Tasks: T11771

Differential Revision: https://secure.phabricator.com/D16739