Network: Don't group mountpoints with different schemes

We're currently accepting mountpoints from different schemes than the previously handled ones.

We incorrectly base ourselves on the device ID only, but since this is usually the hostname, it's not enough to discriminate between a service A exposed by hostname HOST, and a service B also exposed by hostname HOST.

The database needs to be updated to use uuid & scheme as a UNIQUE value, instead of uuid only.

Probing the device also needs to provide the scheme, in order to avoid considering a device as a candidate if it doesn't match the protocol we're watching.