"rndc delzone" is slow, especially on servers with lots of zones,
because the whole file has to be copied (except for one line) every time
a zone is deleted.  It's been measured as having only 20% of the throughput
of addzone.

Proposed solution is to replace the NZF file with a database. I'm inclined to
use berkeley DB for this.

Text NZF files would have to be converted when upgrading, and we also
need a way to convert back to text so the zones in the DB can be inspected.

Since relatively few people really need addzone anyway, it might be better
to leave this as a compile-time option rather than incur an unavoidable
dependency on berkeley DB.