Store TX id with alerts

When generating an alert and storing it in the packet, store the tx_id
as well. This way the output modules can log the tx_id and access the
proper tx for logging.

Issue #904.
Display TX id in alert debuglog.
XFF: use per alert tx id

Use the tx id stored for each alert to find the correct XFF address
to add to the extra-data field.

In overwrite mode we still only grab the first available XFF addr,
as this address is set in the header preceeding the individual alerts.

Issue #904.