Make task selection in syncrepl_update() deterministic.
    
    The fallback task selection logic in syncrepl_update() was
    hidding problems in other parts of code and made event processing
    non-deterministic, which lead to subtle bugs.
    
    Now the fallback is removed so records destined at non-existing zones
    will fail immediatelly instead of failing in later stages or causing
    mayhem.
    
    https://fedorahosted.org/bind-dyndb-ldap/ticket/125