RFR: JDK-8261034: improve jcmd GC.class_histogram to support parallel [v6]

[Chris Plummer]

On Sun, 7 Feb 2021 00:59:00 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:
> 
>   improve jcmd GC.class_histogram to support parallel

diagnosticCommand.hpp still needs a copyright update.

For heapRegionSet.cpp, sorry but you'll need to file a new CR to update its copyright. Among other things, fixing it here makes backports harder to get right.

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Changes requested by cjplummer (Reviewer).

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