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

[Hamlin Li]

parallel -histo of jmap was added by JDK-8214535, it's better to support parallel for jcmd GC.class_histogram too.

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

Changes: https://git.openjdk.java.net/jdk/pull/2379/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jdk&pr=2379&range=00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8261034
  Stats: 7 lines in 2 files changed: 5 ins; 0 del; 2 mod
  Patch: https://git.openjdk.java.net/jdk/pull/2379.diff
  Fetch: git fetch https://git.openjdk.java.net/jdk pull/2379/head:pull/2379

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