RFR: 8326498: java.net.http.HttpClient connection leak using http/2

[Jaikiran Pai]

Can I please get a review for this fix which addresses a connection leak in HttpClient when dealing with HTTP/2 requests?

I have added a comment in https://bugs.openjdk.org/browse/JDK-8326498 which explains what the issue is. The fix here addresses the issue by cleaning up the `Http2Connection` closing logic and centralizing it to a connection terminator. The terminator then ensures that the right resources are closed (including the underlying SocketChannel) when the termination happens.

A new jtreg test has been introduced which reproduces the issue and verifies the fix.

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Commit messages:
 - cleanup
 - update test
 - Return false from isOpen() if underlying channel is not open
 - Daniel's review suggestion - stop the scheduler when TubeSubscriber errored or completed
 - introduce a test to reproduce the leak and verify the fix
 - 8326498: java.net.http.HttpClient connection leak using http/2

Changes: https://git.openjdk.org/jdk/pull/28233/files
  Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=28233&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8326498
  Stats: 960 lines in 9 files changed: 695 ins; 118 del; 147 mod
  Patch: https://git.openjdk.org/jdk/pull/28233.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/28233/head:pull/28233

PR: https://git.openjdk.org/jdk/pull/28233