Motivation:
If a write fails for a Http2MultiplexChannel stream channel, the channel
may be forcibly closed, but only after the promise has been failed. That
means continuations attached to the promise may see the channel in an
inconsistent state of still being open and active.
Modifications:
Move the satisfaction of the promise to after the channel cleanup logic
runs.
Result:
Listeners attached to the future that resulted in a Failed write will
see the stream channel in the correct state.
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