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

[buddyliao(廖彬)]

Hi all,

Would you help me to review the following backport from jdk-master?
The following three patches should be backport together for the reason of the last two is bugfix of the first one.

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

https://bugs.openjdk.java.net/browse/JDK-8251570<https://bugs.openjdk.java.net/browse/JDK-8215624>
https://hg.openjdk.java.net/jdk/jdk/rev/dd827a012e43

https://bugs.openjdk.java.net/browse/JDK-8253763
https://git.openjdk.java.net/jdk/commit/6d19fe65


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

Thank you for Matin’s suggestion, I combined all above three patches together and backport it.

11u webrev:
https://github.com/openjdk/jdk11u-dev/pull/75/files/3bcb48cb64ff0a6dbfa7c6f49f2e08502fab9c22

Testing: x86_64 build, affected tests, tier1

Thanks,
-Buddy