daemon: allow per-request variables in Lua

The handlers in Lua can now store per-request variables that are automatically
GC'd when the request is finished. This is useful for stateful modules,
such as DNS64 that uses internal option flags for state tracking.

The layers can now get a variable table like so:

```
local vars = kres.request_t(r):vars()
vars.hello = true
```

The variables are persisted between different layers for each request.
20 jobs for <span class="ref-name">lua-add-per-request-variables</span> in 11 minutes and 22 seconds (queued for 8 seconds)
Status Job ID Name Coverage
  Build
passed #140931
docker linux amd64
build:clang:linux:amd64

00:00:40

passed #140930
docker linux amd64
build:linux:amd64

00:00:37

passed #140932
docker linux amd64 allowed to fail
srpm

00:00:26

 
  Test
passed #140950
docker linux amd64
deckard:linux:amd64

00:06:33

passed #140946
privileged
distro:epel-7

00:02:35

passed #140945
privileged
distro:fedora-27

00:03:16

passed #140938
docker
doc

00:00:53

passed #140937
docker linux amd64
installcheck:linux:amd64

00:01:13

passed #140940
docker linux amd64
installcheck:valgrind:linux:amd64

00:01:43

passed #140934
docker
lint:c

00:02:48

failed #140935
docker allowed to fail
lint:clang-scan-build

00:01:22

passed #140933
docker
lint:lua

00:00:41

passed #140943
docker linux amd64
respdiff:iter:tcp:linux:amd64

00:03:34

passed #140944
docker linux amd64
respdiff:iter:tls:linux:amd64

00:03:30

passed #140942
docker linux amd64
respdiff:iter:udp:linux:amd64

00:03:37

passed #140936
docker linux amd64
test:linux:amd64

00:01:16

passed #140941
docker linux amd64
test:linux:amd64:valgrind

00:01:18

failed #140939
docker linux amd64
deckard:linux:amd64

00:01:13

failed #140949
docker linux amd64
deckard:linux:amd64

00:01:00

 
  Coverage
passed #140947
docker linux amd64
coverage

00:00:25

69.0%
 
Name Stage Failure
failed
lint:clang-scan-build Test
  CCLD	modules/stats/stats.so
CCLD modules/cookies/cookies.so
CCLD modules/hints/hints.so
scan-build: 28 bugs found.
scan-build: Run 'scan-view /builds/knot/knot-resolver/scan-results/2018-05-30-190142-14-1' to examine bug reports.
Uploading artifacts...
scan-results: found 33 matching files
Uploading artifacts to coordinator... ok id=140935 responseStatus=201 Created token=ydgU6NKY
ERROR: Job failed: exit code 1