Add sensor sample ID to correlate sensor readings

[coderbyheart]

I believe it would be helpful to add a "sample ID" to all sensor readings sent to the cloud, so they can correlated.

For example right now neighbor cell measurements and GNSS position are sent in separate messages (via an MQTT message and via the shadow), with different timestamps which makes it hard to on the cloud side to show the GNSS location the device had when measuring the neighboring cells. Right now this happens by finding a neighboring cell reading with roughly the same timestamp as the GNSS reading, however one needs to decide whether to serch forward or backward, and when to stop searching for a matching reading. This creates imprecise and sometimes faulty correlations.

This sensor sample ID would be generated by the event manager in the application and attached to every sensor reading when being published by the device.

This would also enable a scenario where external sensor readings are stored: imaging we are measuring the performance of the location systems, where the device creates GNSS and cell locations, however using the sample ID being shown by the device on a display we can record the exact geo location on a map by a human and correlate it with the "internal" readings in the cloud.

The sensor sample ID would also be useful in scenarios where the app acts as a controller and other sensor systems report their readings through a different transport.

/cc @simensrostad with whom I discussed this today