RFR: 8252842: Extend jmap to support parallel heap dump [v11]

[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 13 commits:

 - Merge master
 - fix build fail issue on windows
 - hide the dumpheapext error message
 - Merge branch 'master' into pd
 - use parallel= option instead of noparallel
 - Add dumpheapext command for extra argument passing
 - Fix logic in thread number calculation and code refine
 - Merge branch 'master' into par-dump
 - fix buffer queue issue
 - code refine
 - ... and 3 more: https://git.openjdk.java.net/jdk/compare/03d888f4...f0b60d75

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

Changes: https://git.openjdk.java.net/jdk/pull/2261/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jdk&pr=2261&range=10
  Stats: 841 lines in 6 files changed: 654 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