QueueChannel clear() method should be expose in an interface

Sometimes some messages in a QueueChannel will never be consumed because, for instance, timeout of the receiver. Next executions of receive should not consume this old-messages so it would be useful to clear the queue with the clear() QueueChannel method.

XML-configured queue channel can't be retrieve from application context as concrete class QueueChannel without risking of ClassCastException. As clear() method is only declare in concrete class and not in an interface so its not possible to call it from the class that retrieves the queue channel.