Refactor `UDPSocket `API

MR !390 (merged) 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.