tor-proto: Add Notify{Sender,Receiver} channel

An async notification channel. I plan to use this for stream flow control (so that the reactor can notify the DataReader when it has sent an XOFF message), but it's self-contained so I've broken it out into a separate MR.

Alternatively we could have used async-notify, but it's not super popular and I don't see a need to bring in another dependency when we could do the same without much code. I also prefer the sender/receiver API for flow control compared to async-notify and tokio, to make it clear who is notifying who.

This uses postage::watch::{Sender,Receiver} internally.