Clarify Client States in MQTT-SN

The LOST and DISCONNECTED states are in fact the same.

We adopted the session expiry value from MQTT, but this in fact should apply in a different way to the Duration that was attached to the MQTT-SN disconnect. They serve two different functions:

1. Session expiry - how long the server must keep the session state for a client when there is no Virtual Connection. In MQTT terms, while the client is not connected to the server. This is to allow the server to clean up old client session states.
2. Duration, or offline keepalive - the maximum amount of time a client can sleep before the server can delete the Virtual Connection. This allows the server to track clients which should be heard from on a regular basis, and clean up old Virtual Connections.

We have conflated Session Expiry and Duration, because in MQTT there is no notion of a sleeping client - the TCP connection is either open or closed. There is no such limitation in MQTT-SN.

Note that on the above definitions, the Session Expiry timer would come into effect when the Duration/Offline Keepalive timeout occurs. If the Session Expiry timer were 0, then the server could delete the Session immediately.