Simplify ReleephRequest schema

Summary:
Removing a bunch of cache-style columns from ReleephRequest, where it's actually much easier to just load the information at runtime.

This makes sense for migrating to PhabricatorApplicationTransactions, where each xaction changes one aspect of a ReleephRequest at a time (rather than multiple columns at once.)

Test Plan: Request something, run arc releeph and watch the picks, pass on some RQs, run arc releeph and watch the reverts.

Reviewers: wez, epriestley

Reviewed By: epriestley

CC: epriestley, aran

Maniphest Tasks: T2720

Differential Revision: https://secure.phabricator.com/D5851