RFR: 8311867: StructuredTaskScope.shutdown does not interrupt newly started threads

[Alan Bateman]

StructuredTaskScope.shutdown can sometimes not interrupt the thread for a newly forked subtask, leading to join blocking until subtask completes. The "hang" can be duplicated with a stress test that shuts down the scope while a long running subtask is being forked. The bug is in the underlying thread flock code where it filters the threads to just the live threads and so filters out new/unstarted threads, that filtering was left over from some refactoring in the loom repo a long time ago and should have been removed.

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Commit messages:
 - Initial commit

Changes: https://git.openjdk.org/jdk/pull/14833/files
 Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=14833&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8311867
  Stats: 99 lines in 3 files changed: 95 ins; 0 del; 4 mod
  Patch: https://git.openjdk.org/jdk/pull/14833.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/14833/head:pull/14833

PR: https://git.openjdk.org/jdk/pull/14833