Optimise Downloader

Picture Downloader uptimised. It now uses Streams and downloads Tiles
in paralell with a async worker function. Emediatly after completing
a tile download it is added to the mosaic and processd when a whole
picture is downloaded.

 * may need to create the Download stream only when needed.
 * Add a call for the find feautures function, which is not written yet.
 * may add some fields to the DB to handle which images are loaded,
   combined etc.