* plugin/file: respond correctly to IXFR message

Respond with a sing SOA record to an IXFR request if the SOA serials
match.

The added test fails on the current code with:

~~~
=== RUN   TestIxfrResponse
--- FAIL: TestIxfrResponse (0.00s)
    secondary_test.go:122: Expected answer section with single RR
FAIL
exit status 1
~~~

And obviously passes with the new code. This should cut down on the
weird number of zone transfers that I was seeing. At some point IXFR
support might be cool.

Fixes: #3176
Signed-off-by: Miek Gieben <miek@miek.nl>

* reuse code
Signed-off-by: Miek Gieben <miek@miek.nl>

* Sligtht tweaks
Signed-off-by: Miek Gieben <miek@miek.nl>