Move GC into PHP and simplify it

Summary:

• Move GC options into PHP.
• Remove the "run at" and "run for" options. The GC daemon doesn't actually do any table scans, is very gentle, and runs for like 3 seconds per day in any normal install. Just limit it to running once every 4 hours when it's caught up and call it a day.

Test Plan: Edited GC options.

Reviewers: btrahan, codeblock

Reviewed By: codeblock

CC: aran

Maniphest Tasks: T2255

Differential Revision: https://secure.phabricator.com/D4321