The current asynchronous main loop isn't actually asynchronous

The current main loop (run() in onion-tunnel/src/lib.rs), although it's an async fn, isn't actually asynchronous; it calls smoltcp::phy::wait, which is blocking. We should ideally fix this and make it work properly (either by wrapping it in an abstraction that permits blocking, or by refactoring how it works).

note: a MR will fix this very shortly, this is mostly just documenting the problem :p