ipaldap: Override conversion of nsds5replicalast{update,init}{start,end}
    
    The replication related attributes with generalized time syntax have
    special behaviour implemented in 389, as follows:
    
    In case they are explicitly requested for and not set, 0 is returned.
    
    However, 0 is not a valid value for LDAP Generalized time. Thus
    we need to add these attributes to the _SYNTAX_OVERRIDE dictionary,
    overriding their conversion to datetime and converting them to
    string instead, which perserves the old behaviour expected by the
    replication codebase.
    
    https://fedorahosted.org/freeipa/ticket/4350
    
    Reviewed-By: Martin Kosek <mkosek@redhat.com>