RFR: 8371916: Questionable volatile decrement in AckFrameSpliterator

[Daniel Fuchs]

AckFrameSpliterator defines volatile fields. Since Spliterator is not supposed to be MT-safe volatile are not needed there. This change also include some other minor cleanup.

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Commit messages:
 - 8371916: Questionable volatile decrement in AckFrameSpliterator

Changes: https://git.openjdk.org/jdk/pull/28325/files
  Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=28325&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8371916
  Stats: 13 lines in 1 file changed: 0 ins; 0 del; 13 mod
  Patch: https://git.openjdk.org/jdk/pull/28325.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/28325/head:pull/28325

PR: https://git.openjdk.org/jdk/pull/28325