RFR: Sort ShenandoahHeap methods/fields into logical groups

[Roman Kennke]

It's good.

It used to be grouped by 'overidden/implemented methods first' then our
own stuff, but that's probably gone by now, and not that useful anyway.

Push it!
Roman


> http://cr.openjdk.java.net/~shade/shenandoah/reshuffle-sh-heap/webrev.02/
> 
> This reshuffles declarations in shenandoahHeap.hpp from the current mess into logically separate
> subgroups.
> 
> Testing: tier1_gc_shenandoah {fastdebug|release}
> 
> Thanks,
> -Aleksey
>