RFR [M][7/7]: Rebuild remembered sets concurrently

[Thomas Schatzl]

Hi all,

  (last one for now :))

  please have a look at this change that improves TTSP during the
rebuild remembered set phase.

So in the original change G1 yields only after processing a particular
region - in case of humongous objArrays at e.g. 32M region size with
lots of references this can take a bit.

This change modifies the rebuild remembered set closure to yield more
finely grained. Still a single thread processes a single region, there
is a different CR out there for improving that a bit.

The default "chunk size" of 256K of references has been measured to
take <1ms in the large majority of cases on my setup on some of these
densely occupied humongous objArrays, there are log messages for that,
and is configurable.

CR:
https://bugs.openjdk.java.net/browse/JDK-8197932
Webrev:
http://cr.openjdk.java.net/~tschatzl/8197932/webrev/
Testing:
see main thread

Thanks,
  Thomas