Add statuses about optimistic parallel replication stalls.

Hi,

from the documentation [1], I can read:

Non-transactional DML and DDL is not safe to optimistically apply in parallel, as it cannot be rolled back in case of conflicts. Thus, in optimistic mode, non-transactional (such as MyISAM) updates are not applied in parallel with earlier events (it is however possible to apply a MyISAM update in parallel with a later InnoDB update). DDL statements are not applied in parallel with any other transactions, earlier or later.

[1]: https://mariadb.com/kb/en/mariadb/parallel-replication/

It would be very helpful to have slave-side counters (statuses) for both of those non-transnational updates and DDL statements. This would allow to see if optimistic parallel replication is slowed-down by non-transnational updates and DDL statements.

Many thanks,

JFG