Validate configuration and implement negative acknowledgement

[empicano]

When the sensor receives an invalid configuration, it should detect this, stay with its current configuration, and send a negative acknowledgment (success=false) over the heartbeats route back to the server. Updating the software version must be handled as a special case here probably, because it won't necessarily satisfy the validation of the old version.

Neither the dashboard nor the server can implement a reasonable validation for configurations; This has to happen on the sensors.