Bug #904: store tx_id with alerts, update XFF

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.

https://redmine.openinfosecfoundation.org/issues/904
prscript clean: https://buildbot.suricata-ids.org/builders/inliniac/builds/5