Replace TcpSocket with socket2::Socket

[Thomasdezeeuw]

Recently the socket2 crate moved to a new home: https://github.com/rust-lang/socket2-rs, and it is to become the socket implementation for the std lib. For v0.8 I would like to remove the TcpSocket type and instead let users use the Socket type from the socket2 crate.

I personally not looking forward for the stream of prs adding each and every method socket2::Socket already has, such as #1379 and #1375. Since I also maintain socket2, I don't want to maintain the same code twice.

[Thomasdezeeuw]

Not adding socket2 as a dependency, instead we're dropping the TcpSocket.