Support tokio's UdpSocket for DTLS connections in tokio-boring?

I am writing a wrapper for tokio's `UdpSocket` to enable use via `tokio-boring`. There are two use cases for DTLS connections, one with a one-to-one connection where each partner can use `connect()`, and one where a server allows multiple clients to connect and maintains a separate DTLS connection to each client via a single socket. This separation makes a straight-forward implementation of the `AsyncRead`/`AsyncWrite` traits for `UdpSocket` impossible, thus requiring the wrapper.

Would there be interest to add this to tokio-boring? If so, I can clean it up and document it for a PR.