Abstract id tiny patches

4 different patches for rack/session/abstract/id

1. by ivar was renamed to store which I think represents better what is about
2. there's no need to call generate_sid because session.id after doing load! or if the session was already loaded will always return an id https://github.com/spastorino/rack/commit/c751b4747ff7a85d5c1e6c030211ccfa219714a3#L0R318
3. Implement get_session instead of load_session to follow properly the API
4. Fix current_session_id method doc