Handling of single-SOA responses to IXFR request
tests-extra: Responding to IXFR request for newer serial
tests-extra: Minor improvemen
tests-extra: add Response.msg_count and extend Response.count for xfers
tests-extra: Improved test for single-SOA responses.
tests-extra: add simple transfer response check
tests-extra: Added test for master-side AXFR-style IXFR

Also renamed the slave-side test.
tests-extra: move check_axfr_style_ixfr into Response
tests-extra: Improved axfr_style_slave test

- Checking whether master really sends AXFR-style IXFR.
- Also added a convenient wrapper to Test for this.
ixfr: Ignoring malformed IXFR response (old data).
ixfr: Format checking + removed redundant test

- If the IXFR response contains no records or the first record is
  not SOA, consider it malformed and fail the transfer.
- AXFR-style IXFR both master side and slave side are checked in
  one test, removed the other.
ixfr: Fixed format checking.

- Only for the first packet.
- Also restructured the code a bit.
Merge branch 'ixfr-single-soa2' into 'master'

IXFR special cases handling (2)

Fixes #307, see the issue description and the discussion below for details. I went for option A for case (2) as no other people voted. ;-). Case (3) has been modified as proposed by @jvcelak.

See merge request !320