Option to put into slow log any transaction that was retried in parallel replication

In optimistic parallel replication (MDEV-6676), we speculatively apply
transactions in parallel, even if they might conflict. If too many conflicts
happen, excessive retry might degrade performance.

The DBA needs some way to monitor for such problems. One good way would seem
to have an option to add to the slow query log any transactions that were
retried due to parallel replication conflicts. Possibly with extra information
such as duration of transaction and similar.