Allow picking TURN/UDP or TURN/TCP in url params

Also, clean up some legacy TURN-related URL parameters that were
obsolete.  You can now specify a HTTP URL for TURN via ts=, but the
previous ts, tp, ss parameters are now defunct.

PTAL @jiayl, @tkchin
The comment is inaccurate, since turn_base_url is the not the TURN server url, but the url to request the turn servers.