RFR: Sort ShenandoahHeap methods/fields into logical groups

[Aleksey Shipilev]

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