Lock tables for backup blocks DML for RocksDB

Lock tables for backup supposed to block DDL for all storage engines and DML only for non-transactional storage engines. But it turns out that DML for RocksDB is blocked in PS 8.0. I did not check if TokuDB is also affected, nor did I check if 5.7 is affected.

lock instance for backup does not block DML for RocksDB