Update for rust master

`spawn_with` and `spawn_supervised` have been removed in this pull request: https://github.com/mozilla/rust/pull/10603

I also looked up the commit that added `spawn_supervised`: https://github.com/chris-morgan/rust-http/commit/32a28cdc5f77966daeff01f10ed2e10617403133

Seems to be to ensure the server keeps running even if a handler fails. But I've tried randomly inducing failure in the keep-alive loop here, and the parent task seems to keep running just fine.