Make tile downloader check for tile updates

[mikey75]

When downloading a tile, if it is already in cache (memory or 2nd level) it is never checked if the new version is available for download.
If a tile changed on the map source, we should be able to detect it, download the new tile and update cache accordingly. It is however, troublesome, since the web call to api would be needed on every tile draw. Some map services have limits on api calls per hour/day etc. Perhaps we should base it on the tile expiration date in cache.