Merge GC daemon into Trigger daemon

Summary:
Fixes T7352. This reduces the memory footprint for instances by combining these two similar daemons into one daemon which handles the responsibilities of both.

The fit isn't 100% perfect here but it's pretty close, and the GC daemon is fairly trivial.

Test Plan:

• Adjusted all the numbers to small numbers (5 second sleep, 120 second GC length).
• Added a ton of logging.
• Started trigger daemon.
  • Saw it run a GC cycle.
  • Saw it reschedule another cycle after 120 seconds (adjusted down from 4 hours).
• Reverted all the logging/small numbers.
• Ran bin/phd start, saw stable trigger daemon running.
• Grepped for removed daemon class name.

Reviewers: btrahan

Reviewed By: btrahan

Subscribers: epriestley

Maniphest Tasks: T7352

Differential Revision: https://secure.phabricator.com/D11872