Going forward, the config server will always have a shardIdentity document. Currently it uses the fixed name "catalogShard" to avoid special casing for the existing ShardId::kConfigServerId "config." The special casing was necessary when the config server couldn't handle sharded requests, but now the config server will always have the shard components set up, so the special casing should be unnecessary and we can simplify the system by using "config" as the config server's shardIdentity shard name.