RFR: 8286294 : ForkJoinPool.commonPool().close() spins [v5]

[Doug Lea]

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

Doug Lea has updated the pull request incrementally with one additional commit since the last revision:

  Test improvements

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Changes:
  - all: https://git.openjdk.java.net/jdk/pull/8577/files
  - new: https://git.openjdk.java.net/jdk/pull/8577/files/9a0d27f4..4d421ec3

Webrevs:
 - full: https://webrevs.openjdk.java.net/?repo=jdk&pr=8577&range=04
 - incr: https://webrevs.openjdk.java.net/?repo=jdk&pr=8577&range=03-04

  Stats: 30 lines in 2 files changed: 11 ins; 4 del; 15 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