MDEV-38623 ALTER TABLE ... ENGINE = MERGE is binlogged w/o ddl tag

[andrelkin]

The ALTER-ENGINE=MERGE was found out not to have the DDL flag in its Gtid_log_event which is error-prone to the parallel slave. This violates a binlogging requirement that all DDL:s, including all types of ALTER must be binlogged with the flag.

The reason of this exceptional behaviour was an unattended branch of ALTER execution that runs trans_commit_stmt() that cleared already prepared DDL flag from thd->transaction->stmt.m_unsafe_rollback_flags. Later the query gets binlogged stripped from it.

Fixed with applying elsewhere existing pattern (e.g MDEV-8075/228479a28ce7e) to remember-and-restore the original value of m_unsafe_rollback_flags upon the likes of trans_commit_stmt(). This time it's done with arranging a helper StateGuard class to carry out a RAII-like construct as a single-liner.

An mtr test is straightforward and comfortably fits to an existing test file.

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