Garbage collect daemon logs

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