DONT MERGE YET... Review for now

  - Adapted `Store` SPI to current `Cache` API
  - `Ehcache` delegates to `Store` and handles `CacheAccessExceptions`
Keeping this abstract for now, so I don't need to implement the other CRUD methods yet.
Though I'll probably add them in the end, all `throw new UnsupportedOperationException`... at the latest when I wire the `Store` in, instead of having multiple `Cache` implementation as of now...