Automatic MQTT CleanSession flag

[ckrey]

As a learning from #790 we have to consider to manage the CleanSession flag automatically:

• After a configuration change use CleanSession=true on connect
• for all subsequent re-connects, use CleanSession=false
• CleanSession can be remove from configuration

but

• try to recall if any special MQTT-ish brokers required CleanSession=true

[ckrey]

The forced cleanSession should only happen when configuration is changed, not when App is killed intentionally (or killed when phone is restarted)

[ckrey]

We shall keep cleanSession in Settings to allow using cleanSession always

[ckrey]

Release in version 17.3.1