RFR: 8342075: HttpClient: improve HTTP/2 flow control checks

[Daniel Fuchs]

Please find here a fix that improves flow control in the HTTP/2 implementation.

The change makes sure that flow control issues are reported to the server as FLOW_CONTROL_ERROR.
It also clarify how some system properties that allow to initialize flow control windows are handled, by documenting the full range of valid values (when applicable) and explaining what happens if the property points to a value that is out of range.

Bad flow control values in the SETTINGS frame will also cause a FLOW_CONTROL_ERROR to be reported.

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Commit messages:
 - whitespace errors
 - added bug id to test
 - 8342075

Changes: https://git.openjdk.org/jdk/pull/21567/files
  Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=21567&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8342075
  Stats: 1016 lines in 13 files changed: 982 ins; 2 del; 32 mod
  Patch: https://git.openjdk.org/jdk/pull/21567.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/21567/head:pull/21567

PR: https://git.openjdk.org/jdk/pull/21567