Remove `device.connection_metadata` to `device_connection.metadata`

We are moving away from storing connection state on a `device`. We only use `device.connection_metadata` for location-related features; that work should be moved to save/update/read/query off of `device.latest_connection.connection_metadata` instead.

Original conversation in https://github.com/nerves-hub/nerves_hub_web/pull/1807.