[AMQP] non-durable subscription queue not ended on link close

If an AMQP subscriber creates a link to a topic on the broker, then closes that link, the expectation would be that the subscription queue would be deleted at that point if the subscription is not durable, since there is now no client retrieving messages and the queue would just back up.

At present however it seems that the broker will only delete the queue when the connection ends. Though visible in a use case where client is directly connected to the broker, this is particularly an issue when the client's subscription is routed through the router, as in that case the connection may be long lived.