Migrate from Flask to an async framework.

[josiah-wolf-oberholtzer]

Discograph's graph search is always going to be potentially complex. This will become even more crucial before role-based searching goes live, as the number of relations increases by one to two orders of magnitude.

We should migrate to an asynchronous framework to make this as pleasant as possible.

See:

• https://python-aiopeewee.readthedocs.org/en/latest/index.html
• http://tavendo.com/blog/post/going-asynchronous-from-flask-to-twisted-klein/
• http://www.tornadoweb.org/en/stable/