[SCHEMA] Optionally throttle IMAP initial sync for certain accounts.

Summary:
Needs some more testing, but here is the basic gist of it for IMAP. We only
sleep after downloading a message from the initial sync backlog, not after
downloading a message more recent than the sync start. This means that there
may still be up to a minute's latency, but at least you will still get your
new mail if you're getting more than one new message a minute. (It may be
possible to get cleverer.)

Note that we also do one thread per minute rather than one message per minute
on Gmail inbox folders, for simplicity.

Test Plan:
Run syncs for accounts with the 'throttled' flag set to both True
and False. Check that we sleep and output the according debug logging only in
the latter case.

Reviewers: spang

Reviewed By: spang

Differential Revision: https://review.inboxapp.com/D505