RFR: 8298276: JFR: Update NMT events to make use of common periodic timestamp feature

[Erik Gahlin]

Could I have review of a change that ensures NMT events use the same timestamp.

Testing: tier1, tier2, jdk/jdk/jfr/*

Thanks
Erik

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Commit messages:
 - Minor test fix
 - Initial

Changes: https://git.openjdk.org/jdk/pull/12564/files
 Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=12564&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8298276
  Stats: 303 lines in 6 files changed: 143 ins; 158 del; 2 mod
  Patch: https://git.openjdk.org/jdk/pull/12564.diff
  Fetch: git fetch https://git.openjdk.org/jdk pull/12564/head:pull/12564

PR: https://git.openjdk.org/jdk/pull/12564