GEOMESA-605 Refactor mosaicing and GeoServer plugin code

Updated mosaicing code to account for resolution, updated GeoServer plugin code to use the correct URL string, updated GeoServer registration code to properly register raster datastores and layers, fixed an issue with the RasterFilteringIterator breaking on the server-side (thanks to @elahrvivaz), changed mosaicing code to use an Iterator rather than a List