Avoid leaking abstractions of `StateStore`

The `StateStore` interface is user facing and contains a few life-cycle management methods (like `init()` and `close()`) – those methods are exposed for users to develop custom state stores.

However, we also use `StateStore` as base interface for store-handles in the PAPI, and thus life-cycle management methods are leaking into the PAPI (maybe also others – would need a dedicated discussion which one we consider useful for PAPI users and which not).

We should consider to change what we expose in the PAPI (atm, we only document via JavaDocs that eg. `close()` should never be called; but it's of course not ideal and would be better if `close()` et al. would not be expose for `PAPI` users to begin with.)