Make cache events generic where possible

It would be nice if the Cache events were generic, and save the user from needless casting. e.g. CacheEntryActivatedEvent<K, V>
This would also further discourage users from fetching values from their own generic Cache<K, V> rather than having to cast the values from the event or DataContainer (a potential gotcha, see ISPN-703).
Type erasure should ensure that any current unparameterized usage is fully compatible.