Make MyRocks DD commits always durable (facebook#1403)

Summary:
Regular commit durability is governed by the binlog group commit algorithm and system variables. Make any commits (and prepares in 2PC) involving DD tables always durable unconditionally, just like InnoDB does. It is implemented through a new flag `Rdb_transaction::m_dd_transaction`, which is set in `ha_rocksdb::external_lock` and checked in `rocksdb_prepare`. To avoid confusion, rename the existing `m_ddl_transaction` flag to `m_bulk_index_transaction`.

Pull Request resolved: facebook#1403

Differential Revision: D51872251

fbshipit-source-id: 64290c8