Don't try to create IMAP folders before starting sync.

Summary:
This is breaking at least one custom IMAP account sync with `error: create
failed: u'[ALREADYEXISTS] Mailbox already exists'`.

Fixes T571.

Test Plan: Run test sync as sanity-check.

Reviewers: kav-ya

Reviewed By: kav-ya

Maniphest Tasks: T571

Differential Revision: https://review.inboxapp.com/D790