RFR: 8347373: HTTP/2 flow control checks may count unprocessed data twice

[Daniel Fuchs]

The HTTP/2 flow control logic has a potential race condition where some of the unprocessed data may be counted twice for the connection window.
A protocol exception may be raised incorrectly if there are several concurrent streams producing data and the connection window is close from being exhausted.

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Commit messages:
 - 8347373

Changes: https://git.openjdk.org/jdk/pull/23018/files
  Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=23018&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8347373
  Stats: 115 lines in 5 files changed: 49 ins; 4 del; 62 mod
  Patch: https://git.openjdk.org/jdk/pull/23018.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/23018/head:pull/23018

PR: https://git.openjdk.org/jdk/pull/23018