Performance: Improve caches

[famoser]

The caches currently work by Mensa; the menus of each mensa are stored in their own file. This has the advantage that mensas can be refreshed concurrently easily without race conditions (which is indeed currently in use). But this makes the start a bit slower, as many small files have to be read out.

Consider having more coarse-grained caches, for example with a CacheService that manages reads / writes to the cache. This could further improve startup performance.