Use fixed MQTT ID

This is the correct fix instead of the randomness introducted in
f1d44fb.  The "Clean Session" flag tells the MQTT server that we
are a new client and don't wish to inherit any state.  This is an
explicit statement of intention, and more clearly communicates what we
expect to happen to any stale dataflows.

Author: the-maldridge

gizmo.cpp

@@ -565,6 +565,8 @@ void netStateMQTTConnect() {
   }
 
   mqtt.setConnectionTimeout(1000);
+  mqtt.setCleanSession(true);
+  mqtt.setId(cfg.hostname);
   Serial.println("GIZMO_MQTT_TARGET " + cfg.mqttBroker);
   if (!mqtt.connect(mqttIP)) {
     logMQTTError();