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
