Align RocksDB and in-memory store init() sequence

Cf https://lists.apache.org/thread/f4z1vmpb21xhyxl6966xtcb3958fyx5d 

For RocksDB stores, we open the store first and then call #register, [...] However the in-memory store actually registers itself first, before marking itself as open,[..].

I suppose it would make sense to align the two store implementations to have the same behavior, and the in-memory store is probably technically more correct.