RFR: 8252105: parallel heap inspection for ZCollectedHeap [v2]

[Lin Zang]

> - enable parallel heap inspection for ZCollectedHeap
> - preliminary evaluation:
>   Time of jmap histo on 8GB heap with ~5GB objects
>   * before: 7.103s
>   * after : 2.734s (with 4 parallel threads)

Lin Zang has refreshed the contents of this pull request, and previous commits have been removed. The incremental views
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  8252105: parallel heap inspection for ZCollectedHeap
  
  - enable parallel heap inspection for ZCollectedHeap
  - preliminary evaluation:
    Time of jmap histo on 8GB heap with ~5GB objects
    * before: 7.103s
    * after : 2.734s (with 4 parallel threads)

-------------

Changes:
  - all: https://git.openjdk.java.net/jdk/pull/103/files
  - new: https://git.openjdk.java.net/jdk/pull/103/files/b3f33db7..a9b616a5

Webrevs:
 - full: https://webrevs.openjdk.java.net/?repo=jdk&pr=103&range=01
 - incr: https://webrevs.openjdk.java.net/?repo=jdk&pr=103&range=00-01

  Stats: 483 lines in 7 files changed: 116 ins; 343 del; 24 mod
  Patch: https://git.openjdk.java.net/jdk/pull/103.diff
  Fetch: git fetch https://git.openjdk.java.net/jdk pull/103/head:pull/103

PR: https://git.openjdk.java.net/jdk/pull/103