StreamMsg usage cleanup v1.3

For post-beta2:
- big StreamMsg handling cleanup
- small cleanups over #727 
- change vs https://github.com/inliniac/suricata/pull/728 : remove one case where useless smsg was created

Prscript:
- PR build: https://buildbot.suricata-ids.org/builders/inliniac/builds/105
- PR pcaps: https://buildbot.suricata-ids.org/builders/inliniac-pcap/builds/26