Cleanup internal exchanges and queues after disabling plugin/policy

[dcorbacho]

The federation plugins creates internal exchanges, type x-federation-upstream, and queues, noted by the argument federation:x-internal-purpose, to transfer the messages. These internal exchanges and queues are not removed when the the links terminate, either by disabling the plugin or removing the set-upstream policy.

The links ensure that both internal resources are present when they are created, thus there is no reason for them to be left behind. Terminating a link should cleanup all these internal resources.

[michaelklishin]

#65 backported this to 3.6.x.