Summary:
Fixes T9494. This:

  - Removes all the random GC.x.y.z config.
  - Puts it all in one place that's locked and which you use `bin/garbage set-policy ...` to adjust.
  - Makes every TTL-based GC configurable.
  - Simplifies the code in the actual GCs.

Test Plan:
  - Ran `bin/garbage collect` to collect some garbage, until it stopped collecting.
  - Ran `bin/garbage set-policy ...` to shorten policy. Saw change in web UI. Ran `bin/garbage collect` again and saw it collect more garbage.
  - Set policy to indefinite and saw it not collect garabge.
  - Set policy to default and saw it reflected in web UI / `collect`.
  - Ran `bin/phd debug trigger` and saw all GCs fire with reasonable looking queries.
  - Read new docs.

{F857928}

Reviewers: chad

Reviewed By: chad

Maniphest Tasks: T9494

Differential Revision: https://secure.phabricator.com/D14219