remove SDB API and refactor builtin databases

The dns_sdb API is no longer used anywhere except for the builtin databases (empty zones, hostname.bind, version.bind, etc). This MR eliminates it from libdns completely; the code that was needed to implement builtins has been folded into bin/named/builtin.c, which has been refactored so that it implements the dns_db API directly instead of using SDB as a wrapper.

I also incidentally did some cleanup of the dns_db API, making it so that fewer functions are mandatory to implement and so that methods structures are easier to maintain.

Closes #3882 (closed)