RFR: 8252842: Extend jmap to support parallel heap dump [v4]

[Lin Zang]

> 8252842: Extend jmap to support parallel heap dump

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 branch 'master' into par-dump
 - fix buffer queue issue
 - code refine
 - Do not expose parallel thread number to user
 - fix white space trailing issue
 - 8252842: Extend jmap to support parallel heap dump

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Changes:
  - all: https://git.openjdk.java.net/jdk/pull/2261/files
  - new: https://git.openjdk.java.net/jdk/pull/2261/files/fb28c13e..06719159

Webrevs:
 - full: https://webrevs.openjdk.java.net/?repo=jdk&pr=2261&range=03
 - incr: https://webrevs.openjdk.java.net/?repo=jdk&pr=2261&range=02-03

  Stats: 25865 lines in 665 files changed: 9292 ins; 6222 del; 10351 mod
  Patch: https://git.openjdk.java.net/jdk/pull/2261.diff
  Fetch: git fetch https://git.openjdk.java.net/jdk pull/2261/head:pull/2261

PR: https://git.openjdk.java.net/jdk/pull/2261