RFR: 8330734: JFR: Re-engineer mirror class mechanism

[Erik Gahlin]

Could I have a review of a PR that changes how the mirror mechanism works. See issue for details. It reduces startup with 10-15 ms. 

Testing: jdk/jdk/jfr

Thanks
Erik

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Commit messages:
 - Unfix copyright
 - Initial

Changes: https://git.openjdk.org/jdk/pull/18959/files
  Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=18959&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8330734
  Stats: 200 lines in 26 files changed: 41 ins; 73 del; 86 mod
  Patch: https://git.openjdk.org/jdk/pull/18959.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/18959/head:pull/18959

PR: https://git.openjdk.org/jdk/pull/18959