Use a message log to avoid dropping messages

[interfect]

On top of #10, this PR records all the messages from BIND_URL into a log, and then uses that log to answer questions like the current playlist ID.

Actually fetching the playlist data still doesn't work, but it should start to work again whenever a replacement for the watch_queue_ajax endpoint is found. (Doing that probably requires running the requests from the mobile app through an intercepting proxy, since YouTube's Chromecast support in the Chrome browser no longer supports viewing the playlist, and so the browser inspector can't be used to determine the endpoint to call.)