[SPARK-3456] YarnAllocator on alpha can lose container requests to RM

If we expose the number of requests pending to the subclass, we don't need to make this take an extra parameter. E.g.
```
protected def numContainersPending = numPendingAllocate.get()
```
Then the alpha `YarnAllocationHandler#allocateContainers` can add this to `count` instead of the new parameter.