RFR (S/M): 8151069: Parallelize clearing the per-thread concurrent mark data structures

[Thomas Schatzl]

Hi,

  clearing the per-thread concurrent mark data structures single
threaded is a really bad idea since it lenghtens the concurrent mark
cycle.

This change parallelizes this work. It is based on JDK-8077144.

CR:
https://bugs.openjdk.java.net/browse/JDK-8151069
Webrev:
http://cr.openjdk.java.net/~tschatzl/8151069/webrev/
Testing:
jprt, vm.gc testlist, some applications

Thanks,
  Thomas