RFR: 8360046: Scalability issue when submitting virtual threads with almost empty tasks

[Doug Lea]

This set of updates reduces contention-based performance loss under heavy over-subscription, while also improving perfomance more generally.

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Commit messages:
 - Whitespace
 - Merge branch 'openjdk:master' into JDK-8360046
 - Avoid unnecessary use of InterruptibleTask; misc cleanup
 - revert runWorker; tweak external probes
 - reduce contention
 - Merge branch 'openjdk:master' into JDK-8360046
 - For testing
 - Reduce branchiness of Exchanger

Changes: https://git.openjdk.org/jdk/pull/26479/files
  Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=26479&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8360046
  Stats: 142 lines in 3 files changed: 31 ins; 28 del; 83 mod
  Patch: https://git.openjdk.org/jdk/pull/26479.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/26479/head:pull/26479

PR: https://git.openjdk.org/jdk/pull/26479