[Broadcaster] Video sometimes doesn't load

[mickel8]

Sometimes video on the viewer side doesn't load. I would check how we handle keyframe requests sent from the browser

[LVala]

Reason: once in a while, the ICE from our side won't connect, which seems to be caused by packet loss issues (all of the conn checks or responses for these conn checks in a span of 100-200 ms are dropped).

Adding retransmission to ex_ice (elixir-webrtc/ex_ice#56) seems to fix the problem.