optimize: cache NXDomain and reject with 0.0.0.0/::

[mzz2017]

Background

1. When "ipversion_prefer" is on and the domain being looked up does not have an AAAA record, NXDomain was not cached and a new lookup was required each time. It cost much time.
2. Empty answer is NXDomain, which will not be cached by downstream domain server implementation. And a new lookup would be sent to dae each time. It cost many connections/packets.

Modification

1. Cache NXDomain result and non-A/AAAA result.
2. Reject with 0.0.0.0/:: instead of NXDomain.