Also fixed improper promotion of `ffi` to global variable.

```
$ luacheck --codes daemon/lua/
Checking daemon/lua/config.lua                    OK
Checking daemon/lua/kres-gen.lua                  OK
Checking daemon/lua/kres.lua                      OK
Checking daemon/lua/sandbox.lua                   OK
Checking daemon/lua/trust_anchors.lua             OK
Checking daemon/lua/zonefile.lua                  OK

Total: 0 warnings / 0 errors in 6 files
```

The library now depends on libknot >= 2.4.0 (in Debian stable),
this allows us to remove a dead code and sed-ing of the kres.lua

Added a basic config tests to check that constants still work,
and basic interface to rrsets still works after the change.

Fixed predict module, config tests, cleanup

See merge request !399

this tests that:
* sampling frequent queries works
* the code to find periodic appearences of the same name and type
* resolving predicted queries works

it doesn't test pessimistic cases or failure modes

There is no dependency on a testing library yet, so I added a
basic interface for mocking and asserting test values to get
something to start with. I'll probably replace it with busted
or telescope later on to get nicer testing output.

The difficulty with using structs as constant tables is that access
to non-existent fields throws an error. This is difficult to handle
without wrapping every access in a pcall, for example in predict module:

```
error: /usr/local/lib/kdns_modules/predict.lua:34: 'struct rr_type' has no member named 'TYPE65535'
```

So I converted the constant tables into regular Lua tables,
and added a metatable for RR types to allow looking up unnamed types,
in the TYPE%d format. Looking up non-existent fields will now
return nil instead of throwing an error.

The `stats` module is now also included if not exists.

```
$ luacheck modules/predict
Checking modules/predict/predict.lua              OK

Total: 0 warnings / 0 errors in 1 file
```

The config tests locked up on error as if error was raised from the
event callback, it would never reach the `quit()` statement, so
server would never close on error.

Added a script to make running these types of tests a little bit nicer
and to allow concurrent execution of config tests.

Added a test for the predict module, that fails on prediction
of unknown types:

```
error: /usr/local/lib/kdns_modules/predict.lua:34: 'struct rr_type' has no member named 'TYPE65535'
```

Added luacheck for linting Lua files and static analysis

See merge request !398

This is super useful for checking things like misusing undefined
variables or modifying globals, especially in modules when it's
not immediately visible which variables are in the global
namespace and which are not.

I added several exceptions for files in daemon/lua and tests,
as for example sandbox module needs to legitimately modify
global namespace.

There's a lot of things failing, so I didn't make it part of the
standard `make check`, but we should eventually enable it to
improve code quality and spot problems with CI.

CI optimization

Closes #270

See merge request !397

See Deckard MR!85 for details.

Packages are already present in the image so we do not need to spend
time on package maintenance.

For now I didn't touch the generator to be correct on this line,
as we will probably want to change it after bumping minimal libknot
version.

I'm sorry. Buggy commit: 2ba2a5e8

It's very unlikely to happen - sum of string lengths overflowing -
even on a 32-bit platform, and the input seems not controllable by
adversaries, but let's fix it anyway.

Fix check target (check-config requires installed version of kresd)

Closes #272

See merge request !389

Path to module directory gets hardcoded into binaries so check-config
and check-integration cannot work prior installation.

To indicate this depedency, the target check-config was moved from
"check" target to "installcheck".

Deckard requires additional libraries so it is not included in
installcheck.

fixes: #272

Previously it might happen that installed libraries
had higher priority over libraries in build directory.

https://bugs.debian.org/cgi-bin/bugreport.cgi?bug=878976
In general, platforms with page size other than 4k might better compile
with -DCPU_PAGE_SIZE=nnnnn but real impact should only be this test and
better alignment in the mempool allocator.

We can ignore the signal, as the affected libuv calls report error
by returning EPIPE anyway.
Fixes https://gitlab.labs.nic.cz/knot/knot-resolver/issues/271

Add note about initializing submodules needed to build successfully.

See merge request !387

attempt validation for more records but require it for fewer of them
(e.g. avoids SERVFAIL when server adds extra records but omits RRSIGs)

attempt validation for more records but require it for fewer of them
(e.g. avoids SERVFAIL when server adds extra records but omits RRSIGs)