RFR (S/M): 8178105: Switch mark bitmaps during Remark

[Thomas Schatzl]

Hi all,

  can I have reviews for this small change that makes G1 switch the
mark bitmaps already during the Remark pause as opposed waiting until
Cleanup?

This is the last step before G1 can reclaim regions at Remark, which
means that empty regions can be cleaned up more more quickly than
before (JDK-8154528, coming soon :)).

The main changes apart from actually switching the bitmaps consist of
updating the liveness information, i.e. how many bytes are live in a
region, now already available at Remark since JDK-8197850, also at the
same time.

Previously G1 used the Rebuild Remsets phase to calculate that at the
same time.

I could not find significant differences in timing.

This change depends on the recent g1ConcurrentMark refactorings,
particularly JDK-8200234.

CR:
https://bugs.openjdk.java.net/browse/JDK-8178105
Webrev:
http://cr.openjdk.java.net/~tschatzl/8178105/
Testing:
hs-tier 1-5

Thanks,
  Thomas