Protocol fragment tests

[anschweitzer]

Added tests for each of the following protocol fragments:

• Meter aggregate power change
• Meter periodic telemetry
• SimpleSensor periodic telemetry
• Scada 5 minute Status/Snapshot report (also here).
• Atn requests snapshot
• Atn dispatches relay

Protocol changes (per discussion):

• Scada now sends a snapshot with every status and no longer forwards the local dispatch command for the relay to the Atn.
• Turn off BooleanActuator periodic telemetry. It will be replaced by periodic snapshot message from Scada.

Test infrastructure changes:

• Added explicitly logging one line for each message receipt/publication via ScadaSettings.log_message_summary
• Automatically call flush_all() before every test
• Refactor: separate start_mqtt()/stop_mqtt() out of start() and stop() for more controlled testing