* plugin/erratic: add axfr support

Add support for axfr. This to fix and test long standing axfr issues
that are hard to test if we don't support it directly in coredns.

The most intriguing feature is withholding the last SOA from a response
so the client needs to wait; drop (no reply) and delay is also
supported. All TTLs are set to zero.

Add simple tests that checks if first record is a SOA.
Signed-off-by: Miek Gieben <miek@miek.nl>

* more test coverage
Signed-off-by: Miek Gieben <miek@miek.nl>