Missed wakeup events in priority rw lock

Two issues have been identified by stress testing the priority rw lock implementation (not enabled by default):

- high_priority_wait_ex_waiter was never set, meaning, that a sole high-priority X waiter coming after any S waits will not receive its wakeup event;
- Since low-priority S acquisitions were made to wait instead of acquire when a high-priority S lockers exist, it is possible to have waiters on S releases now. They need to be signaled.