RFR: 8286294 : ForkJoinPool.commonPool().close() spins

[Doug Lea]

Changes ForkJoinPool.close spec and code to trap close as a no-op if called on common pool

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Commit messages:
 - Override close as explicit no-op for common pool

Changes: https://git.openjdk.java.net/jdk/pull/8577/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jdk&pr=8577&range=00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8286294
  Stats: 75 lines in 2 files changed: 75 ins; 0 del; 0 mod
  Patch: https://git.openjdk.java.net/jdk/pull/8577.diff
  Fetch: git fetch https://git.openjdk.java.net/jdk pull/8577/head:pull/8577

PR: https://git.openjdk.java.net/jdk/pull/8577