improve contextual logging

[iameli]

It's very difficult these days to use go-livepeer logs to ascertain what's going wrong with a particular broadcast, as most logs.

I propose the following for our logs:

1. Any log that occurs within the context of a stream should have a manifestId listed. Ditto with sessionId for logging that occurs within the context.
2. Make sure all logging uses consistent formatting, e.g. manifestId=ABC123 everywhere.

I'm not sure the best way to make this happen. Maybe we'd start using a context object in more places, so it can get passed down? It'd be good if it could make it all the way down to the LPMS code as well.