[jdk21u-dev] RFR: 8342075: HttpClient: improve HTTP/2 flow control checks

[Alexey Bakhtin]

Please review the backport of JDK-8342075

Almost clean backport except for the import section of src/java.net.http/share/classes/jdk/internal/net/http/Http2Connection.java

Requires JDK-8317808 [1] as a prerequisite to fixing the hanging of the new tests.

Also, new tests require Http2TestExchange::getConnectionKey() method added by JDK-8335181 [2].

test/jdk/java/net/httpclient/lib/jdk/httpclient/test/lib/http2/Http2TestExchange.java, 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 test classes are updated to add  getConnectionKey() implementation

All jtreg tests for java.net.httpclient are passed

[1] - https://github.com/openjdk/jdk21u-dev/pull/1426
[2] - https://github.com/openjdk/jdk/commit/720b44648bcff997278af92746f942b2425298a5#diff-c18c2a070038409f25c08e13c174a95f33840e2deddcc29f6d8306de572a618b

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Depends on: https://git.openjdk.org/jdk21u-dev/pull/1426

Commit messages:
 - Add Http2TestExchange::getConnectionKey() from JDK-8335181
 - Backport b0ac633b2d0076d64b463b2a6ce19abf6b12c50f

Changes: https://git.openjdk.org/jdk21u-dev/pull/1428/files
  Webrev: https://webrevs.openjdk.org/?repo=jdk21u-dev&pr=1428&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8342075
  Stats: 1039 lines in 15 files changed: 1002 ins; 2 del; 35 mod
  Patch: https://git.openjdk.org/jdk21u-dev/pull/1428.diff
  Fetch: git fetch https://git.openjdk.org/jdk21u-dev.git pull/1428/head:pull/1428

PR: https://git.openjdk.org/jdk21u-dev/pull/1428