Use `domain::new::*` in the zone loader.

[ximon18]

The new parser is expected to use less memory and (in future) support parallelized zone loading, plus it's a good way to exercise the new-base and new-zone code in domain.

[bal-e]

In particular, this will require new::zonefile (NLnetLabs/domain#491). It is also related to #307 -- we may want to implement the new loader after a zone storage design is settled.