As the wise man @ywelsch said: currently when we batch cluster state update tasks by the same executor, we the first task un-queued from the pending task queue. That means that other tasks for the same executor are left in the queue. When those are dequeued, they will trigger another run for the same executor. This can give unfair precedence to future tasks of the same executor, even if they weren't batched in the first run. Take this queue for example (all with equal priority)
```
T1 (executor 1)
T2 (executor 1)
T3 (executor 2)
T4 (executor 2)
T5 (executor 1)
T6 (executor 1)
```
If T1 & T2 are picked up first (when T5 & T6 are not yet queued), one would expect T3 & T4 to run second. However, since T2 is still in the queue, it will trigger execution of T5 & T6.
The fix is easy - ignore processed tasks when extracting them from the queue.
Closes#20768
b847a835e9