Use 'cursor' value to determine record position

- When resuming a harvest, it makes much more sense to start
  at the real position rather than 1, so we can determine how
  many records there are left to be fetched.
Simplify check for completion

- We should not trust the 'cursor' position to be correct.
  At least in case of Bibsys, the position is currently implemented
  wrongly, including the number of records in the current batch.