Add a GC for sent and received mail

Summary: Ref T4368. We don't currently GC these tables, and the sent mail table is one of the largest on secure.phabricator.com. There's no value in retaining this information indefinitely. Instead, retain it for 90 days, which should be plenty of time to debug/diagnose any issues.

Test Plan: Ran phd debug garbage, saw it clean up a reasonable amount of data from these tables.

Reviewers: btrahan

Reviewed By: btrahan

CC: aran

Maniphest Tasks: T4368

Differential Revision: https://secure.phabricator.com/D8127