Conpherence - improve reliability of message delivery

Summary:
Ref T7755. T7755#107290 reproduced for me reliably and now it does not. Cleaned up the logic around in flight updates as it was not correct.

Not sure this is enough to close T7755, but maybe?

Test Plan: T7755#107290 no longer reproduces!

Reviewers: epriestley

Reviewed By: epriestley

Subscribers: Korvin, epriestley

Maniphest Tasks: T6713, T7755

Differential Revision: https://secure.phabricator.com/D12755