RFR(M):8252103:parallel heap inspection for ParallelScavengeHeap

[linzang(臧琳)]

Dear All,

        May I ask your help to review this change of adding parallel
heap inspection for PSS Heap?

        bug:   https://bugs.openjdk.java.net/browse/JDK-8252103 

        webrev: 
http://cr.openjdk.java.net/~lzang/jmap-8214535/8252103/webrev.00/

        My preliminary measurement shows about ~1x speed up for when
using 4 threads of jmap -histo. (on a 4 core Linux machine, with -xmx8g
and objects takes 1.5GB in heap.)

        As spaces in youngGen of parallelScvangeHeap is hard to be
divided for parallel iteration, this change just assign different worker
thread to process different space in youngGen, that is, 1 thread to
process whole eden space etc. so this may be one reason that speedup is
not scale with parallel thread number.

BRs,

Lin