RFR: 8252104: parallel heap inspection for ShenandoahHeap [v3]

[Lin Zang]

On Fri, 18 Sep 2020 08:13:53 GMT, Aleksey Shipilev <shade at openjdk.org> wrote:

>> Lin Zang has refreshed the contents of this pull request, and previous commits have been removed. The incremental views
>> will show differences compared to the previous content of the PR.
>
> Sorry for the delay! I think there is a concurrency bug in using `Stack` from multiple threads. Plus, there are a bunch
> of stylistic nits.

Dear @shipilev,
   I have updated the patch, would you like to help review again? Thanks!

-Lin

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PR: https://git.openjdk.java.net/jdk/pull/67