fix: do ECClient disconnect in RTCIceConnectionStateClosed only to prevent crash.

[allenlinli]

I found a bug that it crashes when I disconnect a room.
The bug is described in the issue I sent earlier: #74

I found the reason it crashes is that a block inside ECClient is being released when the ECClient is released(while ECRoom's publishingStatsTimer released, and so that ECRoom and ECClient are released).

So I move the disconnect to RTCIceConnectionStateClosed to prevent this crash.

And after I read the references addressed in this issue (#73), I still didn't find the answer that should both RTCIceConnectionStateDisconnected and RTCIceConnectionStateClosed do disconnect. Since disconnect deallocates ECRoom, and it "probably" always calls RTCIceConnectionStateClosed, we could safely call disconnect at RTCIceConnectionStateClosed.

[allenlinli]

And so far I couldn't reproduce the crash on your sample code yet.
And also I didn't build WebRTC to get the debug symbol see the backtrace (although I guess it's irrelevant). I will try it later.

[zevarito]

thanks @allenlinli! I'll hope to be back on this project next week and review/test all these.