Javadoc
It is private method that looking for U contract in the new items of this slot contract. Then calculates
new payment, looking for already paid, summize it and calculate new prepaid period for storing, that sets to
SlotContract#prepaidKilobytesForDays. This field is measured in the kilobytes*days, means how many kilobytes
storage can hold for how many days.
But if withSaveToState param is false, calculated value
do not saving to state. It is useful for checking set state.data values.
Additionally will be calculated new times of payment refilling, and storing info for previous revision of slot.
It is also useful for slot checking.