GEOMESA-590 Make ClosestAcceptableGeoHash more robust

* Function now returns an Option[GeoHash].
* Modulo check ensures returned GeoHashes are representable.
The Else case of `""` maps the geohash to the entire planet, correct? I don't know what is better: making sure each raster is persisted, but to a silly geohash, or skipping the insertion of these data if the geohash is not acceptable.

@aannex-ccri  what do you think?

Note the latter case is more invasive: you'd have to wrap things in options all the way up to `createMutation`.