net: perform DNS lookup on connect / bind. (#1499)

A sealed `net::ToSocketAddrs` trait is added. This trait is not intended
to be used by users. Instead, it is an argument to `connect` and `bind`
functions.

The operating system's DNS lookup functionality is used. Blocking
operations are performed on a thread pool in order to avoid blocking the
runtime.

tokio-net/tests/tcp_split.rs

@@ -2,7 +2,7 @@ use tokio_net::tcp::{TcpListener, TcpStream};
 
 #[tokio::test]
 async fn split_reunite() -> std::io::Result<()> {
-    let listener = TcpListener::bind(&"127.0.0.1:0".parse().unwrap())?;
+    let listener = TcpListener::bind("127.0.0.1:0").await?;
     let addr = listener.local_addr()?;
     let stream = TcpStream::connect(&addr).await?;
 
@@ -13,7 +13,7 @@ async fn split_reunite() -> std::io::Result<()> {
 
 #[tokio::test]
 async fn split_reunite_error() -> std::io::Result<()> {
-    let listener = TcpListener::bind(&"127.0.0.1:0".parse().unwrap())?;
+    let listener = TcpListener::bind("127.0.0.1:0").await?;
     let addr = listener.local_addr()?;
     let stream = TcpStream::connect(&addr).await?;
     let stream1 = TcpStream::connect(&addr).await?;