Adjust slow-consumer detection logic to use the number of messages in
the queue and not just the number of messages added since the last
check. This means the getRate() method now returns the rate of messages
which it *could* have dispatched since the last check rather than the
rate at which it received messages. This is a more reliable metric to
ensure the slow-consumer detection logic doesn't flag a consumer as
slow unfairly. Although the reliability will come at a performance cost
since getMessageCount() must lock the queue.
Justin Bertram
Andy Taylor