RFR: 8067499: G1SATBCardTableModRefBS should not inherit from CardTableModRefBSForCTRS

[Kim Barrett]

Please review this change to G1SATBCardTableModRefBS to derive from
CardTableModRefBS rather than CardTableModRefBSForCTRS.

Note: I will need a sponsor for this change.

That inheritance change runs afoul of SharedHeap providing remembered set
support.  That seems inappropriate, since the provided remembered set support
is oriented toward generational collection (with incremental update barriers),
but generations show up in GenCollectedHeap, which is derived from SharedHeap.

To address that, the rem_set() member (and associated data member) were moved
from SharedHeap down to GenCollectedHeap.  This permitted some cleanup in the
G1 code, eliminating references to the otherwise vestigal global remembered
set object.  (G1 remembered sets are a different sort of creature.)

A different way of addressing the remembered set location would have been to
change G1CollectedHeap to derive from CollectedHeap rather than SharedHeap,
and that's a change that has been suggested as a technical debt cleanup.  That
change would have required similar changes to G1 code, plus a number of other
changes.  This move of remembered set support can be viewed as a step toward
changing the heap hierarchy.

It may be that the G1 barrier set shouldn't derive from CardTableModRefBS
either; CardTableModRefBS provides a lot of stuff that G1 doesn't use or need,
some of which has to be overridden or called for compatibility.
Alternatively, it might be that some things presently in CardTableModRefBS
should be moved down to CardTableModRefBSForCTRS.  But again, that larger
refactoring would require more changes, and this move of remembered set
support and shift of the G1 barrier set base class can be viewed as a step
toward that larger refactoring.

CR:
https://bugs.openjdk.java.net/browse/JDK-8067499

Webrev:
http://cr.openjdk.java.net/~kbarrett/8067499/webrev

Testing:
Local jtreg of hotspot/test/{runtime,gc}.
JPRT
refworkload with G1, CMS, and ParOld collectors.