RFR: 8252842: Extend jmap to support parallel heap dump [v15]

[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:

  code clean

-------------

Changes:
  - all: https://git.openjdk.java.net/jdk/pull/2261/files
  - new: https://git.openjdk.java.net/jdk/pull/2261/files/1e583904..370e4413

Webrevs:
 - full: https://webrevs.openjdk.java.net/?repo=jdk&pr=2261&range=14
 - incr: https://webrevs.openjdk.java.net/?repo=jdk&pr=2261&range=13-14

  Stats: 1 line in 1 file changed: 0 ins; 1 del; 0 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