Allow discard channel configuration by name in IdempotentReceiverInterceptor

[asarkar]

The IdempotentReceiverInterceptor accepts a MessageChannel bean reference that forces the user to have an @Autowired MessageChannel discardChannel. This is inconsistent with the other patterns that also accept a bean name as a String. I'm suggesting that a method discardChannelName is added such that the user can simply provide a bean name. IdempotentReceiverInterceptor is already BeanFactoryAware, so it can do the lookup just fine.