Consider using `AsyncManualResetEvent` when handling flow state

[lukebakken]

See the following comment from @danielmarbach -

#1643 (review)

Fixed by #1681

[danielmarbach]

I gave this a go since we are still making quite a few breaking changes on the current RC under the umbrella of asyncification. This seems related

[danielmarbach]

I think this can be closed

[lukebakken]

Yessir! Fixed by #1681