RFR (S): Replace VirtualSpace-based pretouch with region-based one

[Aleksey Shipilev]

Hi,

This leverages our existing region infrastructure, and thus makes the whole
thing less susceptible for errors:
  http://cr.openjdk.java.net/~shade/shenandoah/always-pretouch-per-region/webrev.01/

Testing: hotspot_gc_shenandoah, custom runs with release/fastdebug

Thanks,
-Aleksey