Add operational behaviour sections from MQTT 5.0 which we are missing

Section 4 of the MQTT 5.0 specification contains sections we should include,  modified where necessary, in the MQTT-SN specification:

• storing session state
• message ordering
• topic names and topic filters
• subscriptions
• flow control
• server redirection
• enhanced authentication
• handling errors

As far as possible I think we should keep the section ordering the same/similar, for ease of comparison with MQTT 5, and to ensure we don't miss anything out.