Improve connection error handling in IMAP syncback actions.

Summary:
Previously, when executing IMAP syncback actions, we'd catch our custom
ConnectionError/TransientConnectionError exceptions. This makes no sense
whatsoever, because those are only thrown when a connection is authed, and with
connection pooling, connection auth always happens asynchronously in a separate
greenlet.

But what can happen is that the syncback action fails with a socket error; see
T554. Handle this using the same retry_crispin decorator that we use to handle
analogous socket errors in IMAP sync.

Test Plan: System tests.

Reviewers: kav-ya

Reviewed By: kav-ya

Differential Revision: https://review.inboxapp.com/D595