Previously queues were only monitoring channels with subscribers or to
which ack-requiring messages had been delivered. Now queues also
monitor channels from which they have received transactional
publishes.

Queues record the last tx id they have seen from a channel. This then
makes it easy and efficient to find the associated tx record in the
queue's process dictionary when a channel process dies - the
alternative would be to scan the tx records for matching channel pids
- and perform the required rollback activities for the tx.

Passing this around as separate args was becoming a pain. Also, now
it's easier to add more data items.