[jdk17u-dev] RFR: 8347373: HTTP/2 flow control checks may count unprocessed data twice

[Goetz Lindenmaier]

I backport this for parity with 21.0.8-oracle and as follow-up of [JDK-8342075](https://bugs.openjdk.org/browse/JDK-8342075).

I had to do a small resolve in Http2TestServerConnection.java.

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Depends on: https://git.openjdk.org/jdk17u-dev/pull/3852

Commit messages:
 - backport 06126361db1edb1d4c181a82952c1ac133a839f9

Changes: https://git.openjdk.org/jdk17u-dev/pull/3857/files
  Webrev: https://webrevs.openjdk.org/?repo=jdk17u-dev&pr=3857&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/jdk17u-dev/pull/3857.diff
  Fetch: git fetch https://git.openjdk.org/jdk17u-dev.git pull/3857/head:pull/3857

PR: https://git.openjdk.org/jdk17u-dev/pull/3857