RFR: 8312527: (api) Re-examine use of sun.nio.ch.Invoker.myGroupAndInvokeCount

[Brian Burkhalter]

Initialize private static local variable `myGroupAndInvokeCount` to a `ThreadLocal` instead of to an anonymous class effectively identical to `ThreadLocal`.

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Commit messages:
 - 8312527: (api) Re-examine use of sun.nio.ch.Invoker.myGroupAndInvokeCount

Changes: https://git.openjdk.org/jdk/pull/16170/files
 Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=16170&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8312527
  Stats: 7 lines in 1 file changed: 0 ins; 4 del; 3 mod
  Patch: https://git.openjdk.org/jdk/pull/16170.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/16170/head:pull/16170

PR: https://git.openjdk.org/jdk/pull/16170