RFR: 8264001: JFR: Modernize implementation

[Erik Gahlin]

Hi,

Could I have a review of changes that updates the Java implementation of JFR with newer language constructs and library calls.  I also removed unused imports, dead code and added @Override where approriate.

Testing: jdk/jdk/jfr

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Commit messages:
 - Update
 - Update
 - Initial

Changes: https://git.openjdk.java.net/jdk/pull/3138/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jdk&pr=3138&range=00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8264001
  Stats: 346 lines in 57 files changed: 24 ins; 107 del; 215 mod
  Patch: https://git.openjdk.java.net/jdk/pull/3138.diff
  Fetch: git fetch https://git.openjdk.java.net/jdk pull/3138/head:pull/3138

PR: https://git.openjdk.java.net/jdk/pull/3138