ixfr_data_next
struct ixfr_data* next = ixfr_data_next(ixfr, oldest);
data = ixfr_data_next(zone->ixfr, data);
struct ixfr_data* next = ixfr_data_next(ixfr, p);
struct ixfr_data* next = ixfr_data_next(query->zone->ixfr,