flow timeout v2.4

As https://github.com/inliniac/suricata/pull/1269, but with slight commit message rewording.

Ticket https://redmine.openinfosecfoundation.org/issues/1339

Prscript:
- PR build: https://buildbot.openinfosecfoundation.org/builders/inliniac/builds/97
- PR pcaps: https://buildbot.openinfosecfoundation.org/builders/inliniac-pcap/builds/97