Remove unnecessary imports from util/rdb.py.

Summary: Fixes T592.

Test Plan: Run unit test suite.

Reviewers: khamidou

Maniphest Tasks: T592

Differential Revision: https://review.inboxapp.com/D697
Ensure syncs are restarted if necessary after reauth.

Summary:
This is a bit of a stopgap fix for the problem of syncs not being automatically
restarted if they were previously stopped because of invalid credentials, but
the user actually reauths.

Test Plan:
Set test account state to invalid; check that sync gets restarted
after reauth.

Reviewers: kav-ya

Differential Revision: https://review.inboxapp.com/D701
Reject sending to an empty list of recipients.

Previously we'd just reject your post body if you didn't include the recipients
field at all.

Fixes T589.