MR !390 introduced a new `UdpSocket` trait in `tor-rtcompat`. We should consider refactoring it a bit before we make its API semi-stable in our end-of-March release. A couple changes I was thinking of: * Possibly, refactor UdpSocket so that it it is necessarily not "connected". (`ConnectedConnectedUdpSocket` would have to be a separate API, if we someday need it.) * Possibly, refactor `UdpSocket` to implement Stream/Sink rather than using async_trait for sending and receiving.