[jdk11u-dev] RFR: 8215624: Add parallel heap iteration for jmap histo

[BuddyLiao]

8251570: JDK-8215624 causes assert(worker_id <' _n_workers) failed: Invalid worker_id
8253763: ParallelObjectIterator should have virtual destructor

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Commit messages:
 - 8215624: Add parallel heap iteration for jmap histo

Changes: https://git.openjdk.java.net/jdk11u-dev/pull/75/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jdk11u-dev&pr=75&range=00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8215624
  Stats: 290 lines in 14 files changed: 250 ins; 6 del; 34 mod
  Patch: https://git.openjdk.java.net/jdk11u-dev/pull/75.diff
  Fetch: git fetch https://git.openjdk.java.net/jdk11u-dev pull/75/head:pull/75

PR: https://git.openjdk.java.net/jdk11u-dev/pull/75