DB backend for zone.list because zone deletion is slow

[wtoorop]

[wcawijngaards]

We can change the zone list del routine to be faster. The current one can be slow because it attempts to flush and compact on every zone. But this part can be taken out of the loop, at the end of the delzones command. With a large zonelist file the fflush takes some time to complete and on every zone to delete this can add up. By doing the flush only once at the end, it should be much more performant. We can try this can ask if that improves, but I think it will, because actual performance is really a series of small writes, and a flush after it. (But now, flush after writing every couple of bytes, hence slow). Task it then to split the zone_list_del routine in the initial part _mark() and a finish up part that performs the flush and compact check. And then perform those outside the loop in remote.c delzones.

[wcawijngaards]

I think we may also be able to do this for addzones, by the way.