ICE dissolve (legacy)

Two stock RTCPeerConnections whose SDP has its ICE ufrag/pwd rewritten to the shared constants, so ice-dissolve answers the connectivity checks. This path is on its way out: Chrome has announced it will remove ICE credential munging, so a failure here is the browser catching up rather than a server regression. Kept because it is still the clearest way to exercise ice-dissolve from a browser.