Re-introduce the "mark" behavior, to indicate that we've timed out on a request without having received any response from the server. Close the channel when the number of marked stream ids grows too high. This will happen in InFlightHandler.
In 4.x, the request timeout doesn't have any relation to any server timeout, it's set independently from the client's SLA, and will generally be lower IMO. I'm not sure if that affects this ticket.