WL#1874: Thread Priority for Replication

Some users have requested to be able to boost priority of replication (SQL)
thread for replication to go faster even when slave is loaded with significant
SELECT load. 

This should be pretty easy to do but I found the use case for this feature being
very narrow. It only will help on CPU bound loads, where most replication delays
I've seen are due to locking or Disk IO problems.

The other task which might be greater help here is limiting amount of
concurrently executed queries. This would allow to limit disk pressure increasing 
relative number of resources available to replication thread. 

The full solution for this problem of course requires parallel replication.