RFR: 8252842: Extend jmap to support parallel heap dump [v12]

[Lin Zang]

> 8252842: Extend jmap to support parallel heap dump

Lin Zang has updated the pull request incrementally with one additional commit since the last revision:

  Revise jmap help info

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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/f0b60d75..6789ba29

Webrevs:
 - full: https://webrevs.openjdk.java.net/?repo=jdk&pr=2261&range=11
 - incr: https://webrevs.openjdk.java.net/?repo=jdk&pr=2261&range=10-11

  Stats: 17 lines in 1 file changed: 0 ins; 0 del; 17 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