To keep it type-safe, we should bound both the key and value types.

While the union type would probably work, I think this should be kept simple for now.
Something like:
```scala
class CassandraDynamicCompositeStore[RK : CassandraSerializable, CK : CassandraSerializable, V : CassandraSerializable]
  extends Store[(RK, CK), V]
```
A version that supports collections-based composite keys could be added later. What do you think?