Conflicts:
	src/knot/server/zones.c

There was problem with the order of operations - changeset
application modifies the changeset, so it must be done after
the changeset is saved to disk.

Ipv6 Is Not A New Thing

Most is said in the commits.
The basic idea is to reuse the (already provided) structures and streamline in the xfr-handler and server/zone structures in preparation for the zone events.

Conflicts:
	src/knot/server/xfr-handler.c
	src/knot/server/zones.c
	src/knot/server/zones.h

Forwarding Fix

Correct message id reinstating, appending of TSIG RR when forwarding query/response.

Zone Loading Failed Diff + test cases

See the commit messages, should be clear.
Also, the `failed to sign the zone:` message appears also when ixfr-from-differences fails, because it's together in one function (so it prints out even if I don't have DNSSEC signing enabled).
I'm not sure how to handle this properly, maybe have differences and zone signing in a separate functions and print the reason from within.

covered:
- data change, serial increase
- data change, serial doesn't change
- data change, serial < initial serial

zone

fails

Also simplified the operation a lot. First the newly loaded zone
is signed and then the diff is generated (signed zone vs old zone
in memory). Thus we do not need to do changeset merging and other
such magic. The old code was wrong, as it may have lead to a
situation when user makes some changes to DNSSEC records, those are
overwritten by signing the zone, but the changeset contains both
changes resulting in non-consistent zone.

- the zone_master API is actually useful, as it allows rotation for
  multi-master later on

throughout

- it is as good as the sockaddr_t previously, but standard
- subnets are only needed for ACLs
- common way to print addresses (even AF_UNIX)