record pending acks in a queue rather than set in queue process
Just as we do in the channel; this is more efficient for the typical
ack-in-order access pattern.
We depend on tags being passed to the queue process in order when
ack'ing/rejecting. This requires some slightly fiddly code in the
channel.
simplify & document ordering
get rid of #tx{}
and fix uncommitted_acks info item
oops
Merge bug25386