.. this allows a client to properly async publish on connect.

We had been seeing an issue in which the queue could grow larger than the configured cap.  I examined the code and saw that this could happen if _mqttClient.PublishAsync() throws an exception, in which case a message can be re-enqueued without honoring the cap.  Furthermore, I saw that it was possible for the DropOldestQueuedMessage strategy to drop messages that were not actually the oldest ones, because when re-enqueueing the messages in the queue are no longer ordered by the original time they entered the queue.  It made sense to us to peek at the message when publishing rather than dequeue it, so that when re-enqueueing after an exception 1) the cap is still honored and 2) the order of queued messages isn't altered.  It's ok if another thread removes the message that's currently being published from the queue due to the cap, because all we have to do then is check if it's already been removed before removing it ourselves.