Use isctest.asyncserver in the "dnssec" test

Use isctest.asyncserver in the "dnssec" test

Reimplement the custom server using isctest.asyncserver. Factor most of
the logic out of the server code to two (identical) zones and only
implement the non-standard behavior.

Behavior changes:
- RRSIG Expiration and Inception is now static and covering the whole
  timeline instead of being calculated everytime.
- NXDOMAIN is returned where suitable by standard.
- Returned SOA records have properly set zone names.