RFR: 8252842: Extend jmap to support parallel heap dump [v27]

[Lin Zang]

On Mon, 7 Jun 2021 07:58:26 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 36 commits:
> 
>  - Merge branch 'master' into par-dump
>  - Merge branch 'master' into par-dump
>  - fix build issue after rebase and fix one issue
>  - Merge branch 'master'
>  - Merge branch 'master' into pd
>  - update copyright info
>  - Merge branch 'master' into par-dump
>  - undo JMap.java
>  - code refine and typo fix
>  - Merge branch 'master' into par-dump
>  - ... and 26 more: https://git.openjdk.java.net/jdk/compare/908aca29...78c2c763

Dear All， Sorry for not updating this PR for quite a while. I will try to add detailed explaination of the implementation here to make it easier to be reviewed.

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