Native HTTP/2 support

It is possible to get HTTP/2 support by running Xandikos behind a reverse proxy such an apache2 with HTTP/2 support.

Ideally Xandikos would support it natively, but this requires support in aiohttp for HTTP/2 (or a switch to e.g. hyper-h2).

https://github.com/aio-libs/aiohttp/issues/863