RFR: 8252842: Extend jmap to support parallel heap dump [v32]

[Lin Zang]

On Wed, 8 Sep 2021 05:02:18 GMT, Ralf Schmelter <rschmelter at openjdk.org> wrote:

>> Lin Zang has updated the pull request incrementally with one additional commit since the last revision:
>> 
>>   fix build error
>
> I will start reviewing this today.

Thanks @schmelter-sap and @plummercj for your help review and approve this PR!
Since this is a non-trivial change, I would like to wait for 2 days before integrate it in case more comments from others.

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