Merge branch 'cacheevict-optimization' into 'master'

CacheEvict optimization

Currently, in a lot of places the whole cache is cleared when only a single entry changes. Even completely unrelated caches are sometimes unnecessarily cleared.

This PR adds `key` and `condition` elements to the annotations and replaces the `allEntries` element where possible.

There is still room for additional optimizations but these require some refactoring. For now, the related code locations have been commented on to keep track of them.

The last commit of this PR might be obsoleted later by [GH16](https://github.com/thm-projects/arsnova-backend/pull/16). But for now it should massively increase the effectiveness of answer statistics caching.

See merge request !1