RUSTSEC-2020-0016: net2: `net2` crate has been deprecated; use `socket2` instead

[SteveBattista]

https://rustsec.org/advisories/RUSTSEC-2020-0016.html states this.

I found this with cargo audit
https://github.com/RustSec/cargo-audit

It looks like it is in your test rig:
tests/tcp.rs
Line 6 : use net2::TcpStreamExt;
Line 476 let client = net2::TcpBuilder::new_v4().unwrap().to_tcp_stream().unwrap();

[taiki-e]

See #1326, #1319, #1322, #1324, and #1366.