RFR: 8286740: JFR: Active Setting event emitted incorrectly

[Erik Gahlin]

Could I have a review of a fix to the Active Setting event.

- The event object is cached which leads to the event having the same timestamp
- The event is sometimes emitted twice when a recording is started
- Due to caching the event gets a duration, i.e. 0,19 ms. It should be 0 s.

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Commit messages:
 - Initial

Changes: https://git.openjdk.java.net/jdk/pull/8709/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jdk&pr=8709&range=00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8286740
  Stats: 63 lines in 7 files changed: 39 ins; 8 del; 16 mod
  Patch: https://git.openjdk.java.net/jdk/pull/8709.diff
  Fetch: git fetch https://git.openjdk.java.net/jdk pull/8709/head:pull/8709

PR: https://git.openjdk.java.net/jdk/pull/8709