Fixes #12227 - Improve HttpConnection buffer recycling.

[sbordet]

Alternative to #12228.

In this PR, the responsibility to release the buffers is in 2 methods: onFillable() (called when network data is available, and to process the next request) and parseAndFillForContent() (called from Request.read()).

[gregw]

I agree with the approach taken.... just now play whack-a-bug until it works

[gregw]

@sbordet I've been playing whack-a-bug with this, but getting closer....

[gregw]

@sbordet org.eclipse.jetty.test.client.transport.HttpClientIdleTimeoutTest#testRequestIdleTimeout is currently failing because when it idle times out, consumeAvailable is called, which calls parseAndFillForContent, which makes the assumption that if it gets to content end state, then something will call onFillable to do the release. But that is not the case because we are already in the failure mechanism and that is why consumeAvailable has been called. So nothing releases the request buffer.

If I try to add a release in consumeAvailable or parseAndFillForContent then I get NPEs for the cases that think they do need to release the request buffer, but it has already been released.

[sbordet]

@gregw I think the current handling is correct.

It is the test that does not complete the Handler callback, and the check for leaks is performed before the server stops.

I pushed a fix to the tests to complete the callback, and the tests locally pass for me.