Properly update draft objects after sending failure; remove smtpconn_retry.

Summary:
* It doesn't make sense to have retry logic in both the syncback service and in
  the smtpconn_retry decorator; doing so can cause sending actions to get stuck
  for an arbitrarily long amount of time.

* Properly set a draft object's 'state' property to 'sending failed' if we
  can't send it.

Test Plan: Unit tests added.

Reviewers: kav-ya

Reviewed By: kav-ya

Differential Revision: https://review.inboxapp.com/D829