RFR: 8252842: Extend jmap to support parallel heap dump [v18]

[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:

 - remove parallel option and dumpheapext command
 - Revert "hide the dumpheapext error message"
   
   This reverts commit 1af0e1e2bfab4f5d079c03ff0cb25067acacdac4.

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

Changes:
  - all: https://git.openjdk.java.net/jdk/pull/2261/files
  - new: https://git.openjdk.java.net/jdk/pull/2261/files/f0c8a6eb..4f221bd9

Webrevs:
 - full: https://webrevs.openjdk.java.net/?repo=jdk&pr=2261&range=17
 - incr: https://webrevs.openjdk.java.net/?repo=jdk&pr=2261&range=16-17

  Stats: 138 lines in 2 files changed: 0 ins; 136 del; 2 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