RFR (S): 8145774: Move scrubbing setup code away out of ConcurrentMark

[Thomas Schatzl]

Hi all,

  can I have reviews for this change that moves setup and
parallelization detail code for the remset scrubbing code out of
ConcurrentMark into G1CollectedHeap?

This way, ConcurrentMark does not need a direct reference to G1RemSet
and HeapRegionManager any more, and anyway, I think setting up the
gangtask by determining the number of threads to use is better suited to
be placed into G1CollectedHeap.

CR:
https://bugs.openjdk.java.net/browse/JDK-8145774
Webrev:
http://cr.openjdk.java.net/~tschatzl/8145774/webrev/
Testing:
jprt

Thanks,
  Thomas