RFR: 8252842: Extend jmap to support parallel heap dump [v2]

[Lin Zang]

> 8252842: Extend jmap to support parallel heap dump

Lin Zang has updated the pull request incrementally with two additional commits since the last revision:

 - code refine
 - Do not expose parallel thread number to user

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

Changes:
  - all: https://git.openjdk.java.net/jdk/pull/2261/files
  - new: https://git.openjdk.java.net/jdk/pull/2261/files/d6fd93a2..03c0a965

Webrevs:
 - full: https://webrevs.openjdk.java.net/?repo=jdk&pr=2261&range=01
 - incr: https://webrevs.openjdk.java.net/?repo=jdk&pr=2261&range=00-01

  Stats: 221 lines in 4 files changed: 72 ins; 126 del; 23 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