Implement `AsRawFd` for `AddrStream` behind feature flag?

[tedsta]

Hi, I have a use-case where I want to access the file descriptor of a long-lived stream (it becomes a websocket) for monitoring purposes (specifically linux's send-q stat, the amount of unacknowledged outbound bytes) - see here.

Would you be ok with implementing AsRawFd for AddrStream, maybe behind a feature flag? Or otherwise provide a way to get a &tokio::net::TcpStream from a &AddrStream? If it helps and you are open to it I can prepare a PR.

[seanmonstar]

Seems fair and simple to me (Adding AsRawFd support on Linux.)