gunicorn: use async workers

[sevein]

In worker_class, use any of the following options: eventlet, gevent, tornado, gthread. Not considering asyncio because it's Py3 only.
I think it's preferably to use one of the first two options for various reasons. Between the two, I'm inclined to prefer eventlet because it does carry simpler dependencies (eventlet). Ultimately the user could decide which worker class to use.

The reasons for moving to async workers are obvious. One example that makes the underlying issue easier to understand: say you deploy SS with two workers. If one of the workers is busy serving a file to the user, you just compromised 50% of the availability of the server. If the case was that you deployed the application with a single worker, the application will become unavailable to the rest of users. In general, our applications (dashboard and ss) should be able to make use of these worker classes with no changes but some testing is recommended.

[sevein]

Done in #123, using gevent.