RFR: 8252842: Extend jmap to support parallel heap dump [v21]

[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 27 commits:

 - Merge branch 'master' into par-dump
 - refine with several fix
 - Merge branch 'master' into par-dump
 - Merge branch 'master' into par-dump
 - Typo fix
 - remove parallel option and dumpheapext command
 - Revert "hide the dumpheapext error message"
   
   This reverts commit 1af0e1e2bfab4f5d079c03ff0cb25067acacdac4.
 - resize large object threshold
 - Merge branch 'master' into par-dump
 - Merge branch 'master' into par-dump
 - ... and 17 more: https://git.openjdk.java.net/jdk/compare/c3ac6900...6d14790a

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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=20
  Stats: 861 lines in 6 files changed: 653 ins; 63 del; 145 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