RFR: 8272611: Use parallel heap inspection when print classhisto info in gc log [v3]

[Bin Liao]

> This patch helps to enhance the heap inspection with parallel threads when print classhisto info in gc log.
> 
> I have built it on linux x86_64 and run test-tier1 successfully
> 
> ---------
> ### Progress
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Bin Liao has updated the pull request incrementally with one additional commit since the last revision:

  8272611: change heapInspection thread num in ClassHistogramDCmd::execute

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Changes:
  - all: https://git.openjdk.java.net/jdk/pull/5158/files
  - new: https://git.openjdk.java.net/jdk/pull/5158/files/6ae1b554..fa7dbc32

Webrevs:
 - full: https://webrevs.openjdk.java.net/?repo=jdk&pr=5158&range=02
 - incr: https://webrevs.openjdk.java.net/?repo=jdk&pr=5158&range=01-02

  Stats: 3 lines in 2 files changed: 0 ins; 0 del; 3 mod
  Patch: https://git.openjdk.java.net/jdk/pull/5158.diff
  Fetch: git fetch https://git.openjdk.java.net/jdk pull/5158/head:pull/5158

PR: https://git.openjdk.java.net/jdk/pull/5158