Download messages in a thread one-at-a-time.

Summary:
Big threads containing embedded images etc. can use a TON of memory if
we try to download entire threads at a time. Shouldn't actually be much
of a perf loss to download sequentially, and it's way better than
vomiting over everything.

Someday we may want to do something fancier like precalculating message
sizes and using that to chunk.

Test Plan: Ran a sync manually. (no test coverage yet)

Reviewers: emfree

Reviewed By: emfree

Differential Revision: https://review.inboxapp.com/D123