RFR: 8252103: Parallel heap inspection for ParallelScavengeHeap [v2]

[Lin Zang]

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

Lin Zang has refreshed the contents of this pull request, and previous commits have been removed. The incremental views
will show differences compared to the previous content of the PR. The pull request contains one new commit since the
last revision:

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

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Changes:
  - all: https://git.openjdk.java.net/jdk/pull/25/files
  - new: https://git.openjdk.java.net/jdk/pull/25/files/23e09ec4..3672513a

Webrevs:
 - full: https://webrevs.openjdk.java.net/?repo=jdk&pr=25&range=01
 - incr: https://webrevs.openjdk.java.net/?repo=jdk&pr=25&range=00-01

  Stats: 212 lines in 6 files changed: 85 ins; 80 del; 47 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