RFR: 7242: Adding new JDK Events to core API

[Suchita Chaturvedi]

This PR will address these missing events and add them to core API.

jdk.ThreadCPULoad

jdk.NativeMethodSample

jdk.ThreadStart

jdk.ThreadEnd

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Commit messages:
 - Fixed spotbugs errors
 - 7242: Adding new JDK Events to core API

Changes: https://git.openjdk.java.net/jmc/pull/245/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jmc&pr=245&range=00
  Issue: https://bugs.openjdk.java.net/browse/JMC-7242
  Stats: 23 lines in 5 files changed: 21 ins; 0 del; 2 mod
  Patch: https://git.openjdk.java.net/jmc/pull/245.diff
  Fetch: git fetch https://git.openjdk.java.net/jmc pull/245/head:pull/245

PR: https://git.openjdk.java.net/jmc/pull/245