ESC-154 ESC-156 Now using region hash rather than region uuid as identifier.

In the viewer, the region's UUID is acquired very late and isn't generally
used as the canonical region identifier.  The U64 region hash is a better
and more consistently used identifier so I'm moving over to using that as
the region key.  Don't have a proper reserved invalid region hash which is
unfortunate, but then, so much is.