GEOMESA-209 - Identifying unique GeoHash sub-strings in a polygon is bot...

...h slow and inaccurate

Added a few new unit tests, re-wrote the way we compute these
GeoHash sub-strings, and added a few utility methods to the
RectangleGeoHashIterator and the GeoHash.
The compiler might do this rewrite, but just in case...  If you apply DeMorgan's, you give the chance for short-circuit evaluation...