Convert sky_server over to a go-based http server.

We keep seeing timeouts on the bots that seem to be due
to cherrypy dropping requests on the floor, which in turn
causes imports to stall, which causes the test to time out.
In local testing, I was able to reproduce the timeouts
and wasn't able to do so with the go-based server.

R=abarth@chromium.org, eseidel@chromium.org

Review URL: https://codereview.chromium.org/746373002