Message delivery retry is not required where no network failure is detected

WD11 1232 - 1254 Remove existing content in this section and replace with: When a Client reconnects with CleanSession = 0, both the client and server MUST redeliver any previous in-flight messages. This means re-sending any unacknowledged PUBLISH packets (where QoS > 0) and PUBREL packets. This is the only circumstance where a Client or Server is REQUIRED to redeliver messages. Clients MAY resend SUBSCRIBE and UNSUBSCRIBE packets on reconnect but are not REQUIRED to do this. While a modern TCP network is unlikely to lose packets, a Client or Server is permitted to attempt redelivery at other times. However, redelivery is not encouraged unless a network failure has been detected. Non-normative comment Historically retransmission of Control Packets was required to overcome data loss on some older TCP networks. This might remain a concern where MQTT 3.1.1 implementations are to be deployed in such environments.