Enhance loop() to be more resilient

[brentru]

The loop() method seems to cause a lot of confusion regarding its purpose, invocation, and operation. I was discussing with @dhalbert how to improve this and have some ideas...

This issue proposes:

• We create a new method (network_loop(). This method handles:
  • WiFi network connectivity (are we connected prior to polling? What do we do if the WiFi is disconnected during the while True)
  • MQTT broker connectivity (are we still connected to the broker, prior to calling loop()
  • MQTT network loop() polling
  • Handling failures within a MQTT loop() call