RFR: 8238197: JFR: Rework setting and getting EventHandler

[Markus Grönlund]

Greetings,

please help review this changeset which moves the setting and getting of event handlers to native to avoid allocations.

Testing: jdk_jfr, stress

Thanks
Markus

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Commit messages:
 - JFR: Rework setting and getting EventHandler

Changes: https://git.openjdk.java.net/jdk/pull/4445/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jdk&pr=4445&range=00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8238197
  Stats: 92 lines in 2 files changed: 24 ins; 44 del; 24 mod
  Patch: https://git.openjdk.java.net/jdk/pull/4445.diff
  Fetch: git fetch https://git.openjdk.java.net/jdk pull/4445/head:pull/4445

PR: https://git.openjdk.java.net/jdk/pull/4445