Deletes the key from the cache
Returns the expiration time, or undefined if it doesn't exist
Returns the cached value, or undefined if it doesn't exist
Sets the value in the cache, with an optional expiration (as a future Game.time)
Optionalexpiration: numberWraps the cache in a serializer to store objects more compactly. HeapCache notably needs no serializers and simply ignores this.
Cleans up any expired cache values