Minor Downstairs cleanup

[mkeeter]

• tokio::sync::mpsc::Sender is Clone, so there's no need to wrap it in an Arc or Mutex
• We only ever send one message on the terminate_sender queue, so use a tokio::sync::oneshot
• Since we now mix mpsc and oneshot, use qualified names for channel(..), Sender, and Receiver
• Small style tweaks in downstairs/src/main.rs

[faithanalog]

makes sense to me

[leftwo]

Yeah, looks good.

I was a little worried about the interactions between multiple upstairs connecting
at the same time, but the tests cover those cases so it should all be good there.