RFR (M): 8201172: Parallelize Remset Tracking Update Before Rebuild phase

[Thomas Schatzl]

Hi all,

  can I have reviews for this straightforward change that improves the
"Remset Tracking Update Before Rebuild" phase of the Remark pause?

Performance of this phase has not been a big issue, with ~3000 regions
you get around 1ms of time taken, but now it's even faster and
hopefully if run with 30k regions, there are no nasty suprises (or not
as nasty at least).

Determining the number of threads to use has been done by doing a few
measurements and tests and look at which point there does not seem to
be any more scaling (i.e. the noise seems higher than the
improvements), and the pause time much smaller than a millisecond.

CR:
https://bugs.openjdk.java.net/browse/JDK-8201172
Webrev:
http://cr.openjdk.java.net/~tschatzl/8201172/webrev/
Testing:
hs-tier 1-3

Thanks,
  Thomas