Remove dependency on entry->dn to avoid problems with reconstructed entries.
    
    Reconstructed entries might have inaccurate DNs and sometimes DN
    reconstruction is not possible (e.g. for forward zones) because we do
    not have enough information.
    
    Most use cases for entry->dn is in logging so all these cases were
    replaced by new function ldap_entry_logname() which constructs best
    available entry identifier.
    
    The only remaining use case outside ldap_entry module is creating new
    master zone: We need to store zone's DN in zone register for later use
    in dynamic updates.
    
    https://fedorahosted.org/bind-dyndb-ldap/ticket/123