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

[Hamlin Li]

> 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.

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Changes:
  - all: https://git.openjdk.java.net/jdk/pull/2379/files
  - new: https://git.openjdk.java.net/jdk/pull/2379/files/ec2324ec..4584c3a5

Webrevs:
 - full: https://webrevs.openjdk.java.net/?repo=jdk&pr=2379&range=07
 - incr: https://webrevs.openjdk.java.net/?repo=jdk&pr=2379&range=06-07

  Stats: 5 lines in 1 file changed: 1 ins; 0 del; 4 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