feat: support preemption for pods using ReadWriteOncePod PVCs

PVCs using the ReadWriteOncePod access mode can only be referenced by a
single pod. When a pod is scheduled that uses a ReadWriteOncePod PVC,
return "Unschedulable" if the PVC is already in-use in the cluster.

To support preemption, the "VolumeRestrictions" scheduler plugin
computes cycle state during the PreFilter phase. This cycle state
contains the number of references to the ReadWriteOncePod PVCs used by
the pod-to-be-scheduled.

During scheduler simulation (AddPod and RemovePod), we add and remove
reference counts from the cycle state if they use any of these
ReadWriteOncePod PVCs.

In the Filter phase, the scheduler checks if there are any PVC reference
conflicts, and returns "Unschedulable" if there is a conflict.

This is a required feature for the ReadWriteOncePod beta. See for more context:
https://github.com/kubernetes/enhancements/tree/master/keps/sig-storage/2485-read-write-once-pod-pv-access-mode#beta

test/integration/util/util.go

@@ -688,6 +688,16 @@ func CreatePV(cs clientset.Interface, pv *v1.PersistentVolume) (*v1.PersistentVo
 	return cs.CoreV1().PersistentVolumes().Create(context.TODO(), pv, metav1.CreateOptions{})
 }
 
+// DeletePVC deletes the given PVC in the given namespace.
+func DeletePVC(cs clientset.Interface, pvcName string, nsName string) error {
+	return cs.CoreV1().PersistentVolumeClaims(nsName).Delete(context.TODO(), pvcName, *metav1.NewDeleteOptions(0))
+}
+
+// DeletePV deletes the given PV in the given namespace.
+func DeletePV(cs clientset.Interface, pvName string) error {
+	return cs.CoreV1().PersistentVolumes().Delete(context.TODO(), pvName, *metav1.NewDeleteOptions(0))
+}
+
 // RunPausePod creates a pod with "Pause" image and the given config and waits
 // until it is scheduled. It returns its pointer and error status.
 func RunPausePod(cs clientset.Interface, pod *v1.Pod) (*v1.Pod, error) {