list_cleanup()
  Cleanup obsolete records from an SQL list.
  This cleanup is done not more than every 10 minutes to avoid server
  overload.
  
  Usage: list_cleanup(lifetime=36*24*3600, table='greylist')
  
  Where: lifetime is number of seconds after which records are removed (36 days)
         table can define table alternative
  
  Example: list_cleanup()
  
  New in version 0.8.0.