Support for large message transmission

MQTT has an architected limit of 256MB for the message payload. In practice, transmitting a message this large is troublesome. First, the transmission of large messages ties up the network connection for a considerable time, potentially blocking smaller messages. Second, the longer a message takes to transmit, the higher the chance that a network outage will interrupt the transmission, which would then have to be restarted from the beginning. MQTT would make a good transport for delivery of large payloads, in particular device firmware, but only if they can be transmitted in a more manageable way.