RFR: 8179387: Factor out CMS specific code from GenCollectedHeap into its own subclass

[Roman Kennke]

What $SUBJECT says.

I went over genCollectedHeap.[hpp|cpp] and moved everything that I could
find that is CMS-only into a new CMSHeap class.

http://cr.openjdk.java.net/~rkennke/8179387/webrev.00/
<http://cr.openjdk.java.net/%7Erkennke/8179387/webrev.00/>

It is possible that I overlooked something there. There may be code in
there that doesn't shout "CMS" at me, but is still intrinsically CMS stuff.

Also not that I have not removed that little part:

  always_do_update_barrier = UseConcMarkSweepGC;

because I expect it to go away with Erik Ö's big refactoring.

What do you think?

Testing: hotspot_gc, specjvm, some little apps with -XX:+UseConcMarkSweepGC

Roman