def unregister_observer(observer_class, observer_id)
      @observer_mutex.synchronize do
        log "EventListener: un-registering observer #{observer_class} #{observer_id}"
        if (consumer = @observers.delete(observer_key_for(observer_class, observer_id)))
          consumer.cancel
        end
      end
    end
def unregister_observer(observer_class, observer_id)
      @observer_mutex.synchronize do
        log "EventListener: un-registering observer #{observer_class} #{observer_id}"
        if (consumer = @observers.delete(observer_key_for(observer_class, observer_id)))
          consumer.cancel
        end
      end
    end
Unregister an observer instance, and cancel consumption of messages. Any pre-fetched messages will be returned to the queue.