RFR: 8252842: Extend jmap to support parallel heap dump [v17]

[Lin Zang]

> 8252842: Extend jmap to support parallel heap dump

Lin Zang has updated the pull request with a new target base due to a merge or a rebase. The pull request now contains 20 commits:

 - resize large object threshold
 - Merge branch 'master' into par-dump
 - Merge branch 'master' into par-dump
 - code clean
 - fix trailing white space issue
 - reduce memory consumption and fix memory leak issue
 - Revise jmap help info
 - Merge master
 - fix build fail issue on windows
 - hide the dumpheapext error message
 - ... and 10 more: https://git.openjdk.java.net/jdk/compare/d2144a5b...f0c8a6eb

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Changes: https://git.openjdk.java.net/jdk/pull/2261/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jdk&pr=2261&range=16
  Stats: 996 lines in 6 files changed: 796 ins; 58 del; 142 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