Add reason code/string to the disconnect packet

Section should now read; 5.4.21 DISCONNECT Length MsgType ReturnCode SessionExpiryInterval ReasonString (octet 0) (1) (2) (3:6) (7:n) The format of the DISCONNECT message is illustrated in Table 24: Length and MsgType: see Section 5.2. ReturnCode: (optional) Reason for disconnection. See section 5.3.10. Session Expiry Interval: (optional) contains the value of the Session Expiry Interval timer; when the value of this field is greater than zero, it is deemed to be sent by a client that wants to transition to the “asleep” state, see Section 6.14 for further details. ReasonString: (optional) A string representing a clear text description of disconnection. As with MQTT, the DISCONNECT message is sent by a client to indicate that it wants to close the connection. The gateway will acknowledge the receipt of that message by returning a DISCONNECT to the client. A server or gateway may also sends a DISCONNECT to a client, e.g. in case a gateway, due to an error, cannot map a received message to a client. Upon receiving such a DISCONNECT message, a client should try to setup the connection again by sending a CONNECT message to the gateway or server. A DISCONNECT message with a SessionExpiryInterval field is sent by a client when it wants to transition to the “asleep” state. The receipt of this message is also acknowledged by the gateway by means of a DISCONNECT message.