Better handling of waiter invalidation in `PriorityQueue`. (#419)

Add waiter invalidation to prevent abandoned waiters from accumulating
in the `@waiting` heap when exceptions occur during dequeue operations.

- Add `Waiter#invalidate!` method that nullifies fiber and condition.
- Add `Waiter#valid?` method to check if waiter is still usable.
- Add ensure block in dequeue to invalidate waiters on exceptions.