KAFKA-16295: Align RocksDB and in-memory store init() sequences

[ahmedsobeh]

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. This change aligns the RocksDB init sequence with that off the in-memory stores by moving the store opening to be after registration. More context: https://lists.apache.org/thread/f4z1vmpb21xhyxl6966xtcb3958fyx5d

Summary of testing strategy (including rationale) for the feature or bug fix. Unit and/or integration tests are expected for any behaviour change and system tests should be considered for larger changes.

Committer Checklist (excluded from commit message)

• [ ] Verify design and implementation
• [ ] Verify test coverage and CI build status
• [ ] Verify documentation (including upgrade notes)