cgabackup-cleanup: re-enable removal of old/incomplete backups
cgabackup-cleanup: Also remove secondary backups

- when a 2nd backup on a day (e.g. 20140506) is created the original is renamed
  to '20140506a'. Also handle does secondary backups in cgabackup-cleanup.
cgabackup-cleanup: Fix ordering of backups

- Example: 20140506a is older than 20140506
cgabackup-cleanup: also keep backup which is referenced by 'last_backup'