Move PendingApplicationComparator from GuaranteedOrZeroCapacityOverTimePolicy

GuaranteedOrZeroCapacityOverTimePolicy has a comparator class that orders applications by their submit time. It gets the applications from the RMContext and doesn't need any data from GuaranteedOrZeroCapacityOverTimePolicy class, so this easily could be moved to RMContext, so that the reference to the RMContext/SchedulerContext could be removed from GuaranteedOrZeroCapacityOverTimePolicy.