Summary:
We already have GC for daemon log events, but not for daemon logs themselves.
Collect old daemon logs which aren't still running.
Test Plan: Ran `phd debug garbage`, observed old logs get cleaned up. Started some daemons, re-ran garbage, made sure they stuck around.
Reviewers: joshuaspence
Reviewed By: joshuaspence
Subscribers: epriestley
Differential Revision: https://secure.phabricator.com/D9610
