Fix AMATCH FP on multi-TX flows v2

Rebase of https://github.com/inliniac/suricata/pull/891

Prscript:
- PR build: https://buildbot.suricata-ids.org/builders/inliniac/builds/243
- PR pcaps: https://buildbot.suricata-ids.org/builders/inliniac-pcap/builds/163