[mysql-5.6][PR] Make MyRocks DD commits always durable

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: #1403
GitHub Author: Laurynas Biveinis <laurynas.biveinis@gmail.com>

Test Plan: Imported from GitHub, without a `Test Plan:` line.

Reviewers: herman, chni, #mysql_eng

Reviewed By: chni

Differential Revision: https://phabricator.intern.facebook.com/D51872251