fix(server): Enforce serve_connection result usage

[Sol-Ell]

• Mark serve_connection and serve_connection_with_upgrades as must_use to prevent ignored results.
• Mark serve_connection_with_upgrades as must_use for consistency.
• Helps avoid silent issues when forgetting to .await the connection.

Not a major issue, but spent 15 minutes debugging due to a missing .await.

[seanmonstar]

Oh, thanks for PR! Yea, we generally try to provide warnings for people like this. I assumed that must_use on std::future::Future would make it so we didn't need to do add it manually anymore, but it seems that's only for impl Future and Box<dyn Future>.

One request: can they be added to the types themselves, not to the functions, with a note? That's how we've done it for other futures: https://github.com/hyperium/hyper/blob/c449528a33d266a8ca1210baca11e5d649ca6c27/src/server/conn/http1.rs#L38

[Sol-Ell]

Thanks for pointing this out. I'll update everything accordingly.

[seanmonstar]

Lovely, thank you!