[jdk17u-dev] RFR: 8342075: HttpClient: improve HTTP/2 flow control checks

[Alexey Bakhtin]

Please review backport of JDK-8342075

The backport is not clean:

- src/java.net.http/share/classes/module-info.java

skip these changes because there are no java.net.http configuration properties definitions in the file. It was added by JDK-8296804 in JDK20 but not backported

- src/java.net.http/share/classes/jdk/internal/net/http/Stream.java

Comments are not added into Stream::incoming() method because of no JDK-8298340 in JDK17

- src/java.net.http/share/classes/jdk/internal/net/http/WindowUpdateSender.java

manual merge because of differences in the import section caused by JDK-8308310

- src/java.net.http/share/classes/jdk/internal/net/http/Http2Connection.java

Add Http2Connection.java::isOpen method. It was added by 8254786 in JDK19

- test/jdk/java/net/httpclient/lib/jdk/httpclient/test/lib/http2/Http2TestExchange.java  and test/jdk/java/net/httpclient/lib/jdk/httpclient/test/lib/http2/Http2TestExchangeImpl.java and test/jdk/java/net/httpclient/lib/jdk/httpclient/test/lib/http2/Http2TestServerConnection.java are updated from 8335181 with getConnectionKey() implementation

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Commit messages:
 - Use Executor to shutdown httpClient
 - Backport b0ac633b2d0076d64b463b2a6ce19abf6b12c50f

Changes: https://git.openjdk.org/jdk17u-dev/pull/3298/files
  Webrev: https://webrevs.openjdk.org/?repo=jdk17u-dev&pr=3298&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8342075
  Stats: 1058 lines in 14 files changed: 1026 ins; 1 del; 31 mod
  Patch: https://git.openjdk.org/jdk17u-dev/pull/3298.diff
  Fetch: git fetch https://git.openjdk.org/jdk17u-dev.git pull/3298/head:pull/3298

PR: https://git.openjdk.org/jdk17u-dev/pull/3298