Disable autoflush in mailsync.backends.imap.common.update_metadata().
Fixes T413.

Summary:
When we're updating multiple messages in the same thread and a
call to update_thread_labels() results in a Folder.find_or_create(),
a subsequent access of that thread would trigger a flush,
raising an IntegrityError.

Test Plan:
None really as is typical for these errors, ideas welcome.
Sync runs as before.

Reviewers: emfree

Reviewed By: emfree

Differential Revision: https://review.inboxapp.com/D444