Provide bin/garbage for interacting with garbage collection

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.

Reviewers: chad

Reviewed By: chad

Maniphest Tasks: T9494

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