RFR: Cleanup Generational/LRU heuristics code

[Aleksey Shipilev]

Before diving into optimizations, let's cleanup Generational/LRU heuristics code:
  http://cr.openjdk.java.net/~shade/shenandoah/partial-cleanup/webrev.01/

Testing: hotspot_gc_shenandoah

Thanks,
-Aleksey