Eagerly signal sync status.

Summary:
The greenlet executing the sync engine wasn't talking to the sync status
consumer greenlet till the first state transition.
This commit fixes this misbehavior.

Test Plan:
Verifying the previously present bug is kind of tricky, I just noticed it
because I was working on replicating the sync status on Redis.

Reviewers: khamidou, emfree

Reviewed By: emfree

Differential Revision: https://review.inboxapp.com/D698