8195148: Collapse G1SATBCardTableModRefBS and G1SATBCardTableLoggingModRefBS into a single G1BarrierSet

[Erik Österlund]

Hi,

G1 has two barrier sets: an abstract G1SATBCardTableModRefBS barrier set 
that is incomplete and you can't use, and a concrete 
G1SATBCardTableLoggingModRefBS barrier set is what is the one actually 
used all over the place. The inheritance makes this code more difficult 
to understand than it needs to be.

There should really not be an abstract G1 barrier set that is not used - 
it serves no purpose. There should be a single G1BarrierSet instead 
reflecting the actual G1 barriers used.

Webrev:
http://cr.openjdk.java.net/~eosterlund/8195148/webrev.00/

Bug:
https://bugs.openjdk.java.net/browse/JDK-8195148

Thanks,
/Erik