This comes in the form of:
* Per listener maximum_qos option, which can be in the range 0-2.
* Changes to mosquitto_publish*() to return MOSQ_ERR_QOS_NOT_SUPPORTED
if attempting to publish with a higher QoS than supported.
* Bridges will downgrade messages to match the maximum QoS.
More tests on the broker side (specifically bridges) are required. This
needs bridge support for MQTT 5 first.
This change in behaviour can be justified by considering when the
timeout may have occurred.
* If a connection is unreliable and has dropped, but without one end
noticing, the messages will be retried on reconnection. Sending
additional PUBLISH or PUBREL would not have changed anything.
* If a client is overloaded/unable to respond/has a slow connection then
sending additional PUBLISH or PUBREL would not help the client catch
up. Once the backlog has cleared the client will respond. If it is not
able to catch up, sending additional duplicates would not help either.
