RFR: 8285513: JFR: Add more static support for event classes

[Erik Gahlin]

Could I have a review that allows code in java.base, which can't have a compile-time dependency on jdk.jfr, to check if an event can be committed without using an event object. See issue for details.

Testing: 
- jdk/jdk/jfr
- tier1 + tier2
- Tested to implement ThreadSleep event using a static commit method (not part of this changeset)

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Commit messages:
 - Initial

Changes: https://git.openjdk.java.net/jdk/pull/8810/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jdk&pr=8810&range=00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8285513
  Stats: 88 lines in 1 file changed: 72 ins; 14 del; 2 mod
  Patch: https://git.openjdk.java.net/jdk/pull/8810.diff
  Fetch: git fetch https://git.openjdk.java.net/jdk pull/8810/head:pull/8810

PR: https://git.openjdk.java.net/jdk/pull/8810