Minimal support for message hooks and contact ingestion from mail.

Summary:
Add the low-level beginnings of a framework for registering functions
to execute on new mail data, and use it to add contacts from the from/to/cc
fields. Shortly we'll also use this to provide ranking signals for contacts
search.

Test Plan: Unit test added.

Reviewers: mg

Reviewed By: mg

CC: kav-ya

Differential Revision: https://review.inboxapp.com/D96