Fix buffer leak in `HttpClientStreamTest.testUploadWithConcurrentServerCloseClosesStream()`

[lorban]

Depends on https://github.com/jetty/jetty.project/pull/10432 - so this PR is a branch of that other PR.

See #10226

[lorban]

This requires more investigations as stopping the connector should release any pending buffer not yet handed over to user code.

[lorban]

@sbordet nudge

[lorban]

On hold until #10277 is sorted.

[lorban]

The problem is that stopping the connector is notifying the HttpConnection via onClose, but that notification isn't reported to the channel state.

The tentative fix of releasing the buffer in onClose and ensuring the buffer only gets released once by making it atomic isn't right as HttpConnection's buffer shouldn't need any thread safety.

[joakime]

@lorban I think this PR needs to be updated from jetty-12.0.x

[joakime]

@lorban this PR needs a merge from jetty-12.0.x HEAD.

[lorban]

@sbordet nudge

[lorban]

@joakime not that I know. A final review pass from @sbordet would be preferable but I think I addressed all his concerns.