RFR: 8252103: Parallel heap inspection for ParallelScavengeHeap

[Lin Zang]

- Parallel heap iteration support for PSS
- JBS:  https://bugs.openjdk.java.net/browse/JDK-8252103

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Commit messages:
 - 8252103: Parallel heap inspection for ParallelScavengeHeap

Changes: https://git.openjdk.java.net/jdk/pull/25/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jdk&pr=25&range=00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8252103
  Stats: 165 lines in 6 files changed: 165 ins; 0 del; 0 mod
  Patch: https://git.openjdk.java.net/jdk/pull/25.diff
  Fetch: git fetch https://git.openjdk.java.net/jdk pull/25/head:pull/25

PR: https://git.openjdk.java.net/jdk/pull/25