use LRU for packet cache instead of persistent cache

The primary motivation is that packet cache is shared with records,
but it isn't useful for improving performance of popular domain lookups.

Negative answers (with DNSSEC) are typically around 1kB, so it doesn't
take many queries to force a full cache flush, including useful record data.

This changes the packet cache to use a fixed-size LRU, which is more resilient
to random prefix attacks, but it isn't persistent or shared between processes.
18 jobs from marek/lru-cache-packets in 8 minutes and 25 seconds (queued for 9 seconds)
latest
Status Job ID Name Coverage
  Build
passed #137493
docker linux amd64
build:clang:linux:amd64

00:00:39

passed #137492
docker linux amd64
build:linux:amd64

00:00:38

passed #137494
docker linux amd64 allowed to fail
srpm

00:00:24

 
  Test
passed #137501
docker linux amd64
deckard:linux:amd64

00:07:20

passed #137508
privileged
distro:epel-7

00:02:21

passed #137507
privileged
distro:fedora-27

00:03:02

passed #137500
docker
doc

00:00:52

passed #137499
docker linux amd64
installcheck:linux:amd64

00:01:01

passed #137502
docker linux amd64
installcheck:valgrind:linux:amd64

00:01:47

passed #137496
docker
lint:c

00:02:27

failed #137497
docker allowed to fail
lint:clang-scan-build

00:01:25

passed #137495
docker
lint:lua

00:00:36

passed #137505
docker linux amd64
respdiff:iter:tcp:linux:amd64

00:03:35

passed #137506
docker linux amd64
respdiff:iter:tls:linux:amd64

00:03:34

passed #137504
docker linux amd64
respdiff:iter:udp:linux:amd64

00:03:45

passed #137498
docker linux amd64
test:linux:amd64

00:01:15

passed #137503
docker linux amd64
test:linux:amd64:valgrind

00:01:19

 
  Coverage
passed #137509
docker linux amd64
coverage

00:00:22

69.1%
 
Name Stage Failure
failed
lint:clang-scan-build Test
  CCLD	client/kresc
CCLD modules/cookies/cookies.so
CCLD modules/stats/stats.so
scan-build: 28 bugs found.
scan-build: Run 'scan-view /builds/knot/knot-resolver/scan-results/2018-05-16-044411-16-1' to examine bug reports.
Uploading artifacts...
scan-results: found 33 matching files
Uploading artifacts to coordinator... ok id=137497 responseStatus=201 Created token=-J9izBt7
ERROR: Job failed: exit code 1