Configurable OS Scheduling Priority for Slave Threads

Currently, replication worker threads are viewed with the same priority as any other thread in the MariaDB server (and likely any others running on the server). For busy replicas (e.g. serving a lot of reads), non-worker threads will take the same amount of execution time, but don't have the same urgency to complete their work. It would be beneficial to prioritize the worker threads on the underlying operating system so they get more execution time.

Kudos goes to seppo for the idea.