Rewrite router using starlette

[RealOrangeOne]

Replaces https://github.com/dabapps/crab/pull/5

Features:

• Nicely logs the hostname the request is for
• asyncio-based, so has higher throughput, and doesn't get locked down when more than 4 requests come in at once
• Streaming requests, so the router doesn't internally buffer things
• Returns server: crab header
• Uses uvicorn, a production-grade server, rather than a dev server