stream: improve bad window update detection

Ignore more valid ACKs in FIN shutdown phase.

Improve heuristic for window shrinking in case of packet loss.

PRscript:
- PR build: https://buildbot.openinfosecfoundation.org/builders/inliniac/builds/40                 
- PR pcaps: https://buildbot.openinfosecfoundation.org/builders/inliniac-pcap/builds/40