RFR: JDK-8261034: improve jcmd GC.class_histogram to support parallel [v8]

[Hamlin Li]

On Mon, 26 Apr 2021 02:22:47 GMT, Hamlin Li <mli at openjdk.org> wrote:

>> parallel -histo of jmap was added by JDK-8214535, it's better to support parallel for jcmd GC.class_histogram too.
>
> Hamlin Li has updated the pull request incrementally with one additional commit since the last revision:
> 
>   refine help message.

Thanks Lin for reminding.

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PR: https://git.openjdk.java.net/jdk/pull/2379