RFR: 8201171: Cleanup in g1CollectedHeap, change CamelCase to snake_case

[Leo Korinth]

Hi,

I am cleaning up code with regard to style: CamelCase to snake_case.

_cmThread -> _cm_thread
isMarkedNext(oop obj) -> is_marked_next(oop obj)

Bug:
https://bugs.openjdk.java.net/browse/JDK-8201171

Webrev:
http://cr.openjdk.java.net/~lkorinth/8201171/00/index.html

Testing:
- hs-tier1, hs-tier2

Thanks,
Leo