Similar to how authoritative shards is operating, DDL operations will need to begin committing with UUIDs as soon as we enter kUpgrading. However, communications between shards which contain shardRef (uuid or shardId) should not start using UUIDs until fully upgraded. For upgrade, this is likely not strictly necessary but it eases reasoning about shards being aware of new topology times and thus able to resolve shard UUIDs. For downgrade, this gives us some stronger guarantees due to the global lock acquisition during downgrade draining operations which may be choosing between the UUID and shardId.