Wildcard Subsriptions, QoS 1 / 2 optional

["Ken Borgendale" <kwb@us.ibm.com>]

Adding the ability in MQTTv5 to declare that the server does not support QoS levels, wildcard subscriptions, shared subscriptions, or retain does not make something optional which was required before. A server could always not implement these features by just saying that a client using them is not authorized. Such a server was allowed to continue on and just ignore the client request, or to close the network connection without any way of informing the client.

A server which does not support these features now has the ability to declare at connect time that it does not, and to send specific return codes if the client uses these features anyway.

We are looking at including some non-normative text to explain this as part of https://issues.oasis-open.org/browse/MQTT-424

Ken Borgendale -- kwb@us.ibm.com