Add support for write-back to LDAP after a DNS update.
    
    Current code doesn't try to do re-synchronization after an error
    during update. The error is logged and SERVFAIL is returned to
    the client but no re-synchronization is scheduled.
    
    Some hardening against race conditions will be required. Some data could
    be potentially lost if events in syncrepl queue are not processed fast
    enough.
    
    Signed-off-by: Petr Spacek <pspacek@redhat.com>