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

[Chris Plummer]

On Fri, 12 Feb 2021 01:03:28 GMT, Lin Zang <lzang at openjdk.org> wrote:

> So maybe we have two choice here:
>     * Add a new command for argument extension.   so jcmd and jmap could be consisitent.
>     * Remove the "parallel" option in jmap, and leave the control to jcmd.

Yes, I suppose we can still do the `dumpheapext` approach to allow for having 4 args. It's a bit of a hack, but at least it is hidden from the user, and allows `jmap` and `jcmd` to both support `noparallel`

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