Set a timeout for caching of tiles rendered by OSM Scout Server

[M4rtinK]

OSM Scout Server is now pretty robust and even contains automatic map data download & updates.

So it doesn't make sense anymore to store rendered tiles in the cache indefinitely - OSM Scout Server can easily & quickly re-render them. The map data for OSM Scout Server should also be updated rather often, so storing tiles indefinitely also means that they won't ever be re-rendered based on the updated data packs.

So let's set a timeout for how long should OSM Scout Server rendered tiles be considered valid by modRana before it requests them again. I guess 24 hours seems about right ?