Move to tungstenite for WebSockets

[sdroege]

As written at the top of the README.md of websocket:

|Note: Maintainership of this project is slugglish. You may want to use tungstenite or tokio-tungstenite instead.|

[timvisee]

Thanks for the heads-up on this. Good suggestion.

I don't have much time available right now to port to tungstenite though. Feel free to submit a PR.

[werdahias]

Related to timvisee/ffsend#162. +1 for that as it would ease packaging a lot.