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/src/driver/mod.rs

@@ -25,9 +25,7 @@
 //!
 //! # async fn process<T>(t: T) {}
 //! # async fn dox() -> Result<(), Box<dyn std::error::Error>> {
-//! let addr = "93.184.216.34:9243".parse()?;
-//!
-//! let stream = TcpStream::connect(&addr).await?;
+//! let stream = TcpStream::connect("93.184.216.34:9243").await?;
 //!
 //! println!("successfully connected");
 //!