RFR: 8252105: parallel heap inspection for ZCollectedHeap

[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)

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Commit messages:
 - 8252105: parallel heap inspection for ZCollectedHeap

Changes: https://git.openjdk.java.net/jdk/pull/103/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jdk&pr=103&range=00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8252105
  Stats: 380 lines in 6 files changed: 380 ins; 0 del; 0 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