discoverer: Set number to stream infos

The idea is that we can reference to streams using this unique number,
within the context of that discoverer info. That number should always
be usable to reference the streams for a specific stream.

Part-of: <gstreamer/gstreamer!897>
29 jobs for !897 with gst-plugins-base-discoverer-index in 39 minutes and 20 seconds (queued for 7 seconds)
merge request