RFR: 8252842: Extend jmap to support parallel heap dump [v20]

[Lin Zang]

On Wed, 14 Apr 2021 12:35:27 GMT, Lin Zang <lzang at openjdk.org> wrote:

>> 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 25 commits:
> 
>  - 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
>  - code clean
>  - fix trailing white space issue
>  - ... and 15 more: https://git.openjdk.java.net/jdk/compare/e2106d5a...997784b1

Update the PR that refined based on comments from Chris.
@plummercj , thanks very much for your patience reviewing this pr.
 
Testing is still WIP.  I will update the result asap.

BRs,
Lin

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PR: https://git.openjdk.java.net/jdk/pull/2261