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/lib.rs

@@ -38,6 +38,9 @@
 #[macro_use]
 mod tracing;
 
+mod addr;
+pub use addr::ToSocketAddrs;
+
 pub mod driver;
 pub mod util;