RFR: Cleanup SHH::should_start_normal_gc

[Aleksey Shipilev]

http://cr.openjdk.java.net/~shade/shenandoah/cleanups-shh-start-normal/webrev.01/

This cleans up should_start_concurrent_mark:
 - renames to be consistent with partial/traversal
 - eliminates excess arguments that most heuristics do not need
 - purges ShenandoahConcMarkGC flag

Testing: hotspot_fast_gc_shenandoah

Thanks,
-Aleksey