webrtc/nice: synchronous disposal of Nice agent triggers g_warnings when "TURN refreshes" are pending.
Since libnice/libnice!179 (bbf7f2d7) NiceAgent will log a g_warning when trying to dispose a Nice agent with "alive reservations" on a TURN server.
Currently, webrtcbin disposes of the agent synchronously when its state is going down, which can trigger the warnings without the user having any control over that.