Thanks for sharing. Looked over the code, looks solid, and very well organized.
Perhaps would like to see some benchmarks and comparisons with other brokers (including RabbitMQ plugin).
In general with MQTT I was always wondering about its QoS level 2 -- "Once and one once delivery". How well does that work with distributed systems. That would seem a bit hard to implement. (I can see at least once, at most once. Exactly once would be tricky. Thinking about network partitions, disconnects and other such distributed systems shenanigans).