qpid-config lists deleted exchange

In AMQP 0-10, the last exchange to which a message is sent is cached for improved performance. However this means that the exchange object will not be freed when deleted from the registry until all references to it are freed.

To reproduce:

 1. qpid-config add exchange foo
 2. qpid-send -a foo --content-stdin
 3. enter some text to send a message
 4. in another console qpid-config dell exchange foo
 5. qpid-config exchanges

Should not see the deleted exchange in step 5.