client: rework setup request a little

client: rework setup request a little

Cache the media in DESCRIBE based on the longest matching path with the uri
that we can find in the mount points.

Rework the setup request a little to get the media from the session or from
the longest matching path, this way we can derive the control string as
everything after the path instead of hardcoding it.

Find the stream based on the control string and only open a session when all
this can be done.