PS-9451 [8.0] fix unprivileged user login when threadpool is full

https://perconadev.atlassian.net/browse/PS-9452

This patch puts all new connections to the  thread_group->high_prio_queue
instead of normal priority queue to allow unprivileged user login when threadpool
is full.

Author: lukin-oleksiy

sql/threadpool_unix.cc

@@ -993,8 +993,8 @@ static void queue_put(thread_group_t *thread_group, connection_t *connection) {
 
   mysql_mutex_lock(&thread_group->mutex);
   connection->tickets = connection->thd->variables.threadpool_high_prio_tickets;
-  thread_group->queue.push_back(connection);
-
+  //put new connection into high prio queue instead of normal to be able to login
+  thread_group->high_prio_queue.push_back(connection);
   if (thread_group->active_thread_count == 0)
     wake_or_create_thread(thread_group, connection->thd->is_admin_connection());