RFR: 8286688: JFR: Active Setting events should have the same timestamp

[Erik Gahlin]

Could I have a review of a change that ensures that all Active Setting events emitted together get the same timestamp.

Testing: jdk/jdk/jfr + 100 times TestActiveSettingEvent.java

Thanks
Erik

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Commit messages:
 - Initial

Changes: https://git.openjdk.java.net/jdk/pull/8738/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jdk&pr=8738&range=00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8286688
  Stats: 41 lines in 6 files changed: 30 ins; 2 del; 9 mod
  Patch: https://git.openjdk.java.net/jdk/pull/8738.diff
  Fetch: git fetch https://git.openjdk.java.net/jdk pull/8738/head:pull/8738

PR: https://git.openjdk.java.net/jdk/pull/8738