don't hold on to 'complete' in-memory qi journal entries
When a pub, del and ack for a message are all recorded in the journal
then we don't bother writing any of that to the segment files when the
journal is flushed, since the message is well and truly in the past
and forgotten. So... there is no point keeping the entry in the
in-memory journal either, where it just eats up space until a flush
for no good reason.