MosaicRasterSource improvements #344
Description
Right now, the MosaicRasterSource doesn't fetch data from RasterSources asynchornosly. This can cause some serious performance issues in case the mosaic is built of 5 or more RasterSources.
This issues live in the GeoTrellis server repository since it can be a nice playgroud to implement and test the MosaicRasterSource improvement and after that to backport it into the GeoTrellis core.