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

src/docs/user/configuration/managing_daemons.diviner

@@ -70,7 +70,8 @@ You can get a list of launchable daemons with **phd list**:
   - **PhabricatorTaskmasterDaemon** performs work from a task queue;
   - **PhabricatorRepositoryPullLocalDaemon** daemons track repositories, for
     more information see @{article:Diffusion User Guide}; and
-  - **PhabricatorGarbageCollectorDaemon** cleans up old logs and caches.
+  - **PhabricatorTriggerDaemon** schedules event triggers and cleans up old
+    logs and caches.
 
 = Debugging and Tuning =
 
@@ -120,7 +121,7 @@ daemons launch, and split daemons across machines like this:
   - `PhabricatorRepositoryPullLocalDaemon`: Run one copy on any machine.
     On each web frontend which is not running a normal copy, run a copy
     with the `--no-discovery` flag.
-  - `PhabricatorGarbageCollectorDaemon`: Run one copy on any machine.
+  - `PhabricatorTriggerDaemon`: Run one copy on any machine.
   - `PhabricatorTaskmasterDaemon`: Run as many copies as you need to keep
     tasks from backing up. You can run them all on one machine or split them
     across machines.