[11u] RFR 8266352: Add parallel heap iteration for jmap –histo

[buddyliao(廖彬)]

Dear all,

Would you help me to review the following backport from jdk-master?

Original bug:
https://bugs.openjdk.java.net/browse/JDK-8215624
http://hg.openjdk.java.net/jdk/jdk/rev/b1afb7c82d59

Original CSR:
https://bugs.openjdk.java.net/browse/JDK-8266353

Original patch does not apply cleanly to 11u, because jmap command parameter is different between each other, and the code structure has little changed.
Notably, I had to change the src/jdk.jcmd/share/classes/sun/tools/jmap/JMap.java<http://hg.openjdk.java.net/jdk/jdk/file/b1afb7c82d59/src/jdk.jcmd/share/classes/sun/tools/jmap/JMap.java> to make it matches 11u-dev

11u webrev:
  https://cr.openjdk.java.net/~lzang/BuddyLiao/8266352/webrev01/

Testing: x86_64 build, affected tests, tier1

Thanks,
-Buddy