Better behavior on cleanup
    
    With the previous code in domains with many users and enumeration enable we
    would eventually end up making thousands of individual searches for entries in
    the clean-up process.
    
    Change the code to do a full enumeration before a cleanup so we do one single
    big search to update all entries and only then search for entries to purge.
    
    This also fixes the fact that the cleanup task was running at every enumeration
    instead of running every "ldap_purge_cache_timeout" seconds.