RFR: 8252103: Parallel heap inspection for ParallelScavengeHeap [v8]

[Lin Zang]

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

Lin Zang has updated the pull request with a new target base due to a merge or a rebase. The incremental webrev excludes the unrelated changes brought in by the merge/rebase. The pull request contains six additional commits since the last revision:

 - Merge remote-tracking branch 'upstream/master' into jmap-par
 - update the return type of iterable_blocks to size_t
 - cast iterable_blocks to int in claim_and_get_block()
 - fix constant coding style and do code refine
 - Refine HeapBlockClaimer implementation
 - 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/b17c8224..161b5954

Webrevs:
 - full: https://webrevs.openjdk.java.net/?repo=jdk&pr=25&range=07
 - incr: https://webrevs.openjdk.java.net/?repo=jdk&pr=25&range=06-07

  Stats: 485250 lines in 4895 files changed: 398469 ins; 63393 del; 23388 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