[hyperxpro]

Motivation:

HTTP/2 connections live in ChannelManager's registry, not the channel pool, so connectionTtl never applied to them. Nothing else bounds their age: the registry has no reaper and PING keeps an idle connection alive. A client that sets connectionTtl to follow DNS or load-balancer changes keeps every HTTP/2 connection for good.

Modification:

Arm one task per HTTP/2 connection on its event loop when the pipeline is upgraded, delayed by 90-100% of connectionTtl so connections opened together do not expire together. On expiry the connection is retired: it leaves the registry, gives back its ROUND_ROBIN permit, finishes the streams it holds and the openers queued for a slot, and closes once it has neither. Idle check and stream admission are one CAS on the stream count, so a late request either keeps the connection open or is refused before anything is written. A refused request, or one that finds the retired connection already closed, is sent once more through sendNextRequest without spending maxRequestRetry. No GOAWAY is sent early; Netty's close() sends GOAWAY(NO_ERROR) when the idle connection closes.

Result:

connectionTtl bounds HTTP/2 connections as it does HTTP/1.1 ones. With the default of -1 nothing is added to the pipeline.

Fixes #2370