Add support for draining publish events before shutting down.

[nathanielc]

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This change does two things:

1. Adds logic to drain rx_io_requests on shutdown, this ensures that all outgoing requests to the MQTT broken are processed by the IoTask before the task is shutdown.
2. Changes the behavior of a zero read from the client. A zero read does not indicate a disconnect from the broker and so this change simply ignores zero reads and continues processing.

I have been using these changes locally for a few weeks without issue now.