RFR: 8252842: Extend jmap to support parallel heap dump

[Lin Zang]

8252842: Extend jmap to support parallel heap dump

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Commit messages:
 - fix white space trailing issue
 - 8252842: Extend jmap to support parallel heap dump

Changes: https://git.openjdk.java.net/jdk/pull/2261/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jdk&pr=2261&range=00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8252842
  Stats: 777 lines in 6 files changed: 590 ins; 57 del; 130 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