add proof of equivalence of EDF definitions

This commit connects the two ways with which one can specify that a
schedule is an EDF schedule in PROSA: the `EDF_schedule` predicate
and the `respects_policy_at_preemption_point` with the EDF priority
policy predicate. We connect these two definitions by showing that
they're equivalent. We then restate the optimality proof of EDF
schedules using the proven equivalence.