DeviceLister: Support multiple device listers per scheme

In order to fix #305 (closed), we need to be able to register multiple service discovery for a single scheme. For SMB, we need at least microdns and libdms in order to resolve both IP and NetBios names.

However for now, we are storing device lister in a [scheme / instance] map, which doesn't allow us to store more than one device lister. We also assume that 1 scheme will yield 0 or 1 instance, while it should be 0 or N.

An alternative could be to hide more than a single SD module per DeviceLister instance, but it might makes things more confusing