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.
Nice...  As a note, this call was always quick...

The big, bad, slow call was "3-2", i.e. getUniqueGeohashSubstringsInPolygon(poly, 3, 2, 1 << 15)

I haven't read carefully nor tried, but does this approach help with that?