StreamMsg usage cleanup v1.2

For post-beta2:

- big StreamMsg handling cleanup
- small cleanups over https://github.com/inliniac/suricata/pull/727

- PR build: https://buildbot.suricata-ids.org/builders/inliniac/builds/104
- PR pcaps: https://buildbot.suricata-ids.org/builders/inliniac-pcap/builds/25