RFR: 8255471: ZGC: Rework root iterators and closures [v4]

[Stefan Karlsson]

On Fri, 30 Oct 2020 17:07:43 GMT, Erik Österlund <eosterlund at openjdk.org> wrote:

>> Stefan Karlsson has updated the pull request incrementally with one additional commit since the last revision:
>> 
>>   Only restrict nmethod verification when class unloading is turned off
>
> Looks good.

Thanks @fisk

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PR: https://git.openjdk.java.net/jdk/pull/928