Issue3 mbtiles hybrid solution

I removed the background queue and loading should be done on the thread calling `loadMBTilesDataForPath:...`. I also fixed the example app correctly removing the mbtiles layer when switching to a different overlay.

No matter how often I read http://dev.yorhel.nl/doc/sqlaccess, I am still not sure, that my solution is actually better than @wsnook's original implementation, it only takes less memory for sure. It probably boils down to how often MapKit creates new threads for loading tiles.