handle incoming stream redirection

[iameli]

We need to handle incoming requests for e.g. https://playback.livepeer.studio/hls/PLAYBACK_ID/index.m3u8 and 302 redirect them to the address of a Catalyst server that has the stream in question. Previously, we proxied all of these requests behind the scenes, but that led to some suboptimal performance characteristics, especially around segment download latencies.

The workflow will be basically:

1. Handle GET https://playback.livepeer.studio/hls/PLAYBACK_ID/index.m3u8
2. Turn around and ask MistUtilLoad for the best place to watch the pertinent stream, like so:

▶ curl 'http://localhost:8042/video+PLAYBACK_ID'
185.102.219.180

3. 302 Redirect the user to that URL https://185.102.219.180/hls/PLAYBACK_ID/index.m3u8
4. If no stream is present, 404

There will be some nuance in getting this right, I think... for example, we'll want to check for the X-Forwarded-Proto: https header and return an HTTPS url if present, that sort of thing.

@iameli TODO: Document why we're doing the flow this way