proxy: replace net2 with socket2

[olix0r]

The proxy depends on the net2 crate, which has been deprecated and replaced by socket2. We should update our one or two uses:

linkerd/app/Cargo.toml
42:net2 = "0.2"

linkerd/app/integration/Cargo.toml
32:net2 = "0.2"

linkerd/app/integration/src/server.rs
205:        let listener = net2::TcpBuilder::new_v4().expect("Tcp::new_v4");

linkerd/app/integration/src/controller.rs
337:    let listener = net2::TcpBuilder::new_v4().expect("Tcp::new_v4");

[hawkw]

As a side note, this isn't gonna be totally banished from our dependency tree until Tokio migrates from mio 0.6 to mio 0.7 (tokio-rs/tokio#2677) a breaking change scheduled for Tokio 0.3. But, I imagine that we can remove our uses of it (which all appear to be in the integration tests).

[hawkw]

I actually have no idea why the integration tests don't just use tokio::net::TcpListener::bind...there is probably some backstory for this...

[hawkw]

Ah, the test change from TcpListener::bind to socket2 was in #952, because we added a test that required introducing a delay between binding and listening on the socket. So, we do still need this, and can probably just switch to socket2.