RFR: 8302821: JFR: Periodic task thread spins after recording has stopped

[Erik Gahlin]

Could I have a review of a PR that sleeps the periodic task thread once JFR is stopped.

Testing: jdk/jdk/jfr

Thanks
Erik

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Commit messages:
 - Initial

Changes: https://git.openjdk.org/jdk/pull/12702/files
 Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=12702&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8302821
  Stats: 15 lines in 1 file changed: 15 ins; 0 del; 0 mod
  Patch: https://git.openjdk.org/jdk/pull/12702.diff
  Fetch: git fetch https://git.openjdk.org/jdk pull/12702/head:pull/12702

PR: https://git.openjdk.org/jdk/pull/12702