Http events v2

Convert HTTP decoder event handling to be TX-aware: https://redmine.openinfosecfoundation.org/issues/839
Fix 2 other small issues.

Prscript:
- PR build: https://buildbot.suricata-ids.org/builders/inliniac/builds/181
- PR pcaps: https://buildbot.suricata-ids.org/builders/inliniac-pcap/builds/101