Summary:
This diff adds persistent unique ids for messages and contacts. For messages, we just take a hash of the headers. For contacts, we hash the contact's email address.
This diff bundles a couple of tiny fixes too, like always trying to restart an account's sync, even after an exception.
Note that since there's no reliable way to have persistent ids for threads, we'll have to change our code to use message ids instead. Alas, that's a story for another (massive) diff.
Test Plan: Tested manually.
Reviewers: bengotow
Reviewed By: bengotow
Differential Revision: https://phab.nylas.com/D3468
