This is the bulk of the CI/CD overhaul.

Most of the changes are to the `.gitlab-ci.yml` file, where the build
images used are replaced with the ones provided by the
`knot-resolver-ci` repository. Some cleanups have also been done.

The commit also adds unit testing with Knot Resolver built against
multiple versions of Knot DNS, including the `master` branch. The
`master` branch image is built nightly in the `knot-resolver-ci` repo.

We have also removed `scan-build`, as its tests change frequently, with
lots of false-positives, which are very different on each version, and
there is no good way to ignore some detections. Clang-Tidy covers some
of the same issues, and we also have Coverity Scan. Should be more than
enough.

A few config tests were also excluded in the AddressSanitizer tests,
because they produce false-positives.