Replace Sensor and Actuator Pin property with immutable Source property

[Alan Noble]

The Sensor and Actuator “Pin” property is formed by concatenating a device ID with a pin, i.e., “ESP_ABCDEF.A0”. It would be preferable if sensors and actuators instead used the ID of the data source, i.e., a scalar ID or media ID, which are immutable. The suggested name for the new property is “Source”, since it is the source of the data on which the sensor/actuator operates.

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[Alan Noble]

Sensors and Actuators now use a compound key, which is the concatenation of the corresponding device's MAC address and the pin.