MDEV-10297 Add priorization to threadpool

Also MDEV-10385 Threadpool refactoring

mysql-test/r/mysqld--help.result

@@ -1105,6 +1105,15 @@ The following options may be given as the first argument:
  --thread-pool-oversubscribe=# 
  How many additional active worker threads in a group are
  allowed.
+ --thread-pool-prio-kickup-timer=# 
+ The number of milliseconds before a dequeued low-priority
+ statement is moved to the high-priority queue
+ --thread-pool-priority=name 
+ Threadpool priority. High priority connections usually
+ start executing earlier than low priority.If priority set
+ to 'auto', the the actual priority(low or high) is
+ determined based on whether or not connection is inside
+ transaction.
  --thread-pool-size=# 
  Number of thread groups in the pool. This parameter is
  roughly equivalent to maximum number of concurrently
@@ -1461,6 +1470,8 @@ thread-cache-size 151
 thread-pool-idle-timeout 60
 thread-pool-max-threads 1000
 thread-pool-oversubscribe 3
+thread-pool-prio-kickup-timer 1000
+thread-pool-priority auto
 thread-pool-stall-limit 500
 thread-stack 297984
 time-format %H:%i:%s