- Right now, the serial is bumped N times for every API call, which is
really not great when you consider that systemd-networkd and others
like it are caching name resolvers.
- This can be elided by checking if a matching record exists in the
catalog before inserting it.
- Tests were added. Fixtures in some spots were created. Read
`src/tests.rs` before trying tests.
- Code was heavily reorganized as a result.
Signed-off-by: Erik Hollensbe <linux@hollensbe.org>