- first phase is preemptive (prepare_update_keyspace)
- second phase is non-preemptive (update_keyspace)
This is done so that schema change can be applied atomically.
Aditionally create keyspace code was changed to share common
part with update keyspace flow.
This commit doesn't yet change the behaviour of the code,
as it doesn't guarantee atomicity, it will be done in following
commits.
2daa630938