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

[Lin Zang]

On Wed, 3 Feb 2021 12:44:53 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.

Hi @Hamlin-Li 
I am not reviewer, but maybe you could add test for "parallel" in BasicJMapTest.java.

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