[SCHEMA] Establish an association between messages and contacts.

Summary:
We want this mostly so that we can filter messages based on the from/to/cc/bcc
fields -- constructing SQL queries against the current JSON-encoded fields is
extremely clunky.

This commit also removes the HookManager abstraction, which we probably won't
use since we're not doing synchronous hooks.

Test Plan: Unit tests updated.

Reviewers: spang, mg

Reviewed By: spang

Differential Revision: https://review.inboxapp.com/D127