Parallel Replication to Support MySQL 8+ Master

A MariaDB replica cannot be configured to use parallel replication if it is consuming from a master that is MySQL 8.0 or higher. This can be problematic for migration, as replication wouldn't be able to keep up with busy workloads. The current workaround for this is to perform migrations during a period of low expected traffic.

The main factor that prevents this is that the MariaDB 10.0+ primary embeds its binlog events with metadata that the replica uses to make scheduling decisions (e.g. for DDLs and non-transactional transactions), which the MySQL 8+ master doesn't include.