[SPARK-3411] Improve load-balancing of concurrently-submitted drivers across workers

If the waiting driver array is too big, the drivers in it will be dispatched to the first worker we get(if it has enough resources), with or without the Randomization.

We should do randomization every time we dispatch a driver, in order to better balance drivers.
I think it's sufficient to say something like

We assign workers to each waiting driver in a round-robin fashion. For each driver, we start from the last worker that was assigned a driver, and continue onwards until we have explored all alive workers.