Currently the cql statement .validate() callback is responsible for
checking if the non-local storage options are allowed with the
respective feature. Next patch will need to extend this check to also
validate the details of the provided storage options, but doing it at
cql level doesn't seem correct -- it's "too far" from query processor
down to sstables manager.
Good news is that there's a lower-level validation of the new keyspace,
namely the database::validate_new_keyspace() call. Move the storage
options validation into sstables manager, while at it, reimplement it
as a visitor to facilitate further extentions and plug the new
validation to the aforementioned database::validate_new_keyspace().
Signed-off-by: Pavel Emelyanov <xemul@scylladb.com>