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.
[nitpick]  Since this function is producing a seq from internal state rather than converting a collection, I don't think I'd call it .toSeq.