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.
As a sanity check, is an instance of this Object instantiated each time the function is called?  A quick test in the REPL seems to say it would....