RFR: 8256641: CDS VM operations do not lock the heap [v2]

[Thomas Schatzl]

On Wed, 9 Dec 2020 23:02:54 GMT, Ioi Lam <iklam at openjdk.org> wrote:

>> Thomas Schatzl has updated the pull request incrementally with one additional commit since the last revision:
>> 
>>   kbarrett review
>
> Marked as reviewed by iklam (Reviewer).

I fixed the mentioned comments but would like to defer further cleanup of the classes, particularly those `VM_GC_Operation`s that do not actually participate in the skipping protocol to [JDK-8258029](https://bugs.openjdk.java.net/browse/JDK-8258029) I filed just now.

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PR: https://git.openjdk.java.net/jdk/pull/1661