WebRTC plugin doesn't kill ExWebRTC processes during shutdown

[FelonEkonom]

Steps to reproduce:

1. Enter boombox project, execute iex -S mix command and run Boombox.run(input: {:webrtc, "ws://localhost:8829"}, output: {:hls, "index.m3u8"})
2. Enter WebRTC from browser html page from boombox_examples_data via path in Google Chrome
3. Click Connect in the web page, wait few seconds and click Disconects
4. Now you should see new logs from ex_webrtc in the Elixir shell from time to time.