R1:9a8eb4026e42

Make race condition window for Conpherence smaller and rarer

Summary:
Ref T6713. This isn't very clean, and primarily unblocks D11143.

After D11143, I have a reliable local race where I submit, get a notification immediately, then get a double update (form submission + notification-triggered update).

Instead, make the notification updates wait for form submissions.

This doesn't resolve the race completely. The notification updates don't block chat submission (only the other way around), so if you're…