Unify API

[sehz]

In the async-native-tls, there is a single TlsStream, but in this crate, there are two different Stream implementation. It would be nice if we have unified API that can support different engine.

[skade]

I'm trying to remember what the exact cause for having both streams was. I tried unifying both of them a while back, but if I remember right, it's at least not trivial, at least without dynamic dispatch.

That said, I'd prefer one TLS implementation for all runtimes.