RFR: 8252842: Extend jmap to support parallel heap dump [v37]

[Lin Zang]

> 8252842: Extend jmap to support parallel heap dump

Lin Zang has refreshed the contents of this pull request, and previous commits have been removed. The incremental views will show differences compared to the previous content of the PR. The pull request contains one new commit since the last revision:

  fix build issue

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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/43c516a4..fcc2674c

Webrevs:
 - full: https://webrevs.openjdk.java.net/?repo=jdk&pr=2261&range=36
 - incr: https://webrevs.openjdk.java.net/?repo=jdk&pr=2261&range=35-36

  Stats: 6 lines in 1 file changed: 0 ins; 1 del; 5 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