RFR: 8069367: assert(_nextMarkBitMap->isMarked((HeapWord*) obj)) failed

[Kim Barrett]

Another round.

Replaced add/remove_candidate with setter for candidate set.  Replaced
add/remove_humongous_reclaim_candidate modifiers with setter for
G1CollectedHeap.  These changes allow some code simplifications in the
candidate selection.

Share common arguments for initializing _in_cset_fast_test and
_humongous_reclaim_candidates, rather than recomputing for each.

Improved various comments based on review feedback.  
 
CR:
https://bugs.openjdk.java.net/browse/JDK-8069367 

Webrev:
http://cr.openjdk.java.net/~kbarrett/8069367/webrev.05/

Incremental webrev:
http://cr.openjdk.java.net/~kbarrett/8069367/webrev.05.incr/

Testing:
Hand testing
JPRT
local RefWorkload using G1