[rfc] dns memcap v0.2

Continuation of https://github.com/inliniac/suricata/pull/787. Now with counters.

Pscript:
- PR build: https://buildbot.suricata-ids.org/builders/inliniac/builds/153
- PR pcaps: https://buildbot.suricata-ids.org/builders/inliniac-pcap/builds/73