(processing locks RCU per-response, but zone content must not change
 during the iterations)

fixes #215

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.