Non-transactional mysql.gtid_slave_pos breaks --slave-parallel-mode=[optimistic|aggressive] replication

See mail thread at https://lists.launchpad.net/maria-developers/msg09722.html.

Optimistic parallel replication does not work when mysql.gtid_slave_pos is
non-transactional. So either it needs to fallback to non-parallel (like for
other non-transactional updates/DDL), or an error should be given.

An error seems better, as silently falling back to all transactions being
run serially does not seem very useful.