RFR: 8277397: ZGC: Add JFR event for temporary latency measurements [v3]

[Stefan Karlsson]

> I often measure latencies and stalls using JFR events. I'd like to add an event that can be used for these ad-hoc measurements during development and debugging.

Stefan Karlsson has updated the pull request incrementally with one additional commit since the last revision:

  Rename ZThreadEvent to ZThreadDebug

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Changes:
  - all: https://git.openjdk.java.net/jdk/pull/6454/files
  - new: https://git.openjdk.java.net/jdk/pull/6454/files/478c38a4..15f4d9a7

Webrevs:
 - full: https://webrevs.openjdk.java.net/?repo=jdk&pr=6454&range=02
 - incr: https://webrevs.openjdk.java.net/?repo=jdk&pr=6454&range=01-02

  Stats: 34 lines in 6 files changed: 0 ins; 18 del; 16 mod
  Patch: https://git.openjdk.java.net/jdk/pull/6454.diff
  Fetch: git fetch https://git.openjdk.java.net/jdk pull/6454/head:pull/6454

PR: https://git.openjdk.java.net/jdk/pull/6454