RFR: add parallel heap inspection support for jmap histo(G1)

[臧琳]

Dear All,
     May I ask your help to review the follow changes:
     webrev: http://cr.openjdk.java.net/~lzang/jmap-8214535/8215264/webrev_00/
     bug: https://bugs.openjdk.java.net/browse/JDK-8215624
     related CSR: https://bugs.openjdk.java.net/browse/JDK-8239290
     This patch enable parallel heap inspection of G1 for jmap histo.
     my simple test shown it can speed up 2x of jmap -histo with 
parallelThreadNum set to 2 for heap at ~500M on 4-core platform.

________________________________
BRs,
Lin