Switch to using prefetch rather than credit, that way we don't have to freeze the basic.credit methods. This has some minor implications:

 * The basic.get hack is no longer possible, so we need to take it out. But everyone seemed to hate it anyway...
 * We no longer limit the rate of message ingress in no-ack mode. But that's what we do for federated exchanges.
 * The code gets a bit simpler. Woo.