rb://498
  
  Fix handling of update_undo_logs at trx commit. Previously, when
  rseg->update_undo_list grows beyond 500 the update_undo_logs were
  marked with state TRX_UNDO_TO_FREE which should have been
  TRX_UNDO_TO_PURGE.
  In 5.5 we don't need the heuristic as we support multiple rollback
  segments.
  
  Approved by: Sunny Bains