Each query that is not already resolved is forwarded to the specified
server for resolution, and the response is sent back.

Forwarder uses TCP only (needs merge of resolver_compat).

Knotc doc

See merge request !285

knotc: dynamic buffer for response

See merge request !284

closes #298

Additional nsec null checks

See merge request !283

fix race during server reload

closes #296

See merge request !281

refs #296

refs #296

closes #293

Axfr ixfr client

See merge request !280

This reverts commit e774c8a8.

IXFR: falback to AXFR master-side fix

When Knot master replied to IXFR query by AXFR-style IXFR, the response was wrong in two ways:
1. Header was cleared: no ID, no QR flag
2. QTYPE was changed to AXFR

Both are wrong. The header must be set properly and the IXFR QTYPE must be preserved even if the response has the AXFR format (that's the way to distinguish it), see [RFC1995, Section 4](http://tools.ietf.org/html/rfc1995#section-4).

I removed these changes to the packet. But I'm not totally sure that there is no problem now, because the packet-processing is too obscure. @mvavrusa should review this definitely.

Tests pending.

See merge request !278

- Response had the header cleared (no ID and no QR flag).
- Also, the Question was wrongly set to AXFR. It should remain IXFR
  although the response is in AXFR format. (See RFC1995, Section 4)