UnboundedRabbitMqReader can fail to advance watermark if no new data comes in

I stumbled upon this and then saw a similar StackOverflow post: https://stackoverflow.com/questions/55736593/apache-beam-rabbitmqio-watermark-doesnt-advance

When calling `advance()` if there are no messages, no state changes, including no changes to the CheckpointMark or Watermark.  If there is a relatively constant rate of new messages coming in, this is not a problem. If data is bursty, and there are periods of no new messages coming in, the watermark will never advance.

Contrast this with some of the logic in PubsubIO which will make provisions for periods of inactivity to advance the watermark (although it, too, is imperfect: https://issues.apache.org/jira/browse/BEAM-7322 )

The example given in the StackOverflow post is something like this:

pipeline
  .apply(RabbitMqIO.read()
  .withUri("amqp://guest:guest@localhost:5672")
  .withQueue("test")
  .apply("Windowing", 
    Window.<RabbitMqMessage>into(
      FixedWindows.of(Duration.standardSeconds(10)))
    .triggering(AfterWatermark.pastEndOfWindow())
    .withAllowedLateness(Duration.ZERO)
    .accumulatingFiredPanes())

If I push 2 messages into my rabbit queue, I see 2 unack'd messages and a window that never performs an on time trigger.