amqp: set the auto_delete bit on queues.

amqp: set the auto_delete bit on queues.

Without this, AMQP queues are leaked whenever we reconnect to an ircd.
Exclusive queues are only deleted when the connection is torn-down;
auto-deleted queues are also deleted when the last (i.e. only in this case)
conusmer is removed.

Since the AMQP consumers are tied to processes running under the irc_object_sup
for the IRC connection, these will be torn-down when we reconnect to the ircd.