RFR: Rename and cleanup _regions and _free_set uses

[Aleksey Shipilev]

http://cr.openjdk.java.net/~shade/shenandoah/rename-or-fs/webrev.01/

Before we go and do more FreeSet work, we need to make a few cosmetic fixes:
 *) Drop "active_regions" from everywhere. It stopped making sense after we did heap uncommit
mechanics, which made all regions available in the region set at all times;
 *) "ordered_regions" name is too verbose, of course they are ordered;
 *) "num_regions()" and "capacity()" are used where applicable, without polling the region set

Testing: hotspot_gc_shenandoah

Thanks,
-Aleksey