refs !205

Both master and slave scenarios. Master should fail, slave should
load the zone.

Later, log messages should be checked too, but that's not possible now.

Query plan modules + synthetic PTR/A/AAAA records

close #166

Conflicts:
	src/knot/updates/xfr-in.c
	src/knot/zone/zone-create.c
	src/knot/zone/zone-create.h

Gcov

This MR adds support to enable code coverage testing with:

```shell
./configure --enable-code-coverage
make
make check-code-coverage
```

The output can be found in $(PACKAGE_NAME)-$(PACKAGE_VERSION)-coverage directory.

You have to have lcov package installed for this to work.

DNSKEY in non-apex nodes

This changeset improves handling of DNSKEY records in non-apex nodes.