Add positive cache timeout

[xxv]

So that images that get cached are updated, standard HTTP caching techniques should be employed to ensure that they aren't out of date.

This includes conditional HTTP headers, e-tag storing and last-modified storing. The metadata should be stored in a way that hitting the "clear cache" button in the system UI will clear the database too.

It may even make sense to look into using a proper HTTP caching library for this part, while still using this for any resized derivatives. This library will still need to know when to refresh derivatives, so it may need to communicate with the cache.