RFR: 8252842: Extend jmap to support parallel heap dump [v13]

[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:

  reduce memory consumption and fix memory leak issue

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

Changes:
  - all: https://git.openjdk.java.net/jdk/pull/2261/files
  - new: https://git.openjdk.java.net/jdk/pull/2261/files/6789ba29..d636a140

Webrevs:
 - full: https://webrevs.openjdk.java.net/?repo=jdk&pr=2261&range=12
 - incr: https://webrevs.openjdk.java.net/?repo=jdk&pr=2261&range=11-12

  Stats: 200 lines in 2 files changed: 159 ins; 14 del; 27 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