Bug #904: store tx_id with alerts, update XFF -- v1.1

Store TX id in the packet, so easy access to it by output modules. Update alert-debuglog to output this info.

Update the XFF output in unified2 to try to use this tx_id for retrieving the XFF ip.

Bug: https://redmine.openinfosecfoundation.org/issues/904
prscript: https://buildbot.suricata-ids.org/builders/inliniac/builds/19

Replaces https://github.com/inliniac/suricata/pull/555.

Changes:
- address comments from @serializingme
- rebase against master