feat(client): add Resolve, used by HttpConnector

This introduces a `Resolve` trait to describe asynchronous DNS
resolution. The `HttpConnector` can be configured with a resolver,
allowing a user to still use all the functionality of the
`HttpConnector`, while customizing the DNS resolution.

To prevent a breaking change, the `HttpConnector` has its `Resolve`
generic set by default to `GaiResolver`. This is same as the existing
resolver, which uses `getaddrinfo` inside a thread pool.

Closes #1517

src/client/connect/mod.rs

@@ -15,6 +15,7 @@ use tokio_io::{AsyncRead, AsyncWrite};
 
 #[cfg(feature = "runtime")] mod dns;
 #[cfg(feature = "runtime")] mod http;
+#[cfg(feature = "runtime")] pub use self::dns::{GaiResolver, Name, Resolve};
 #[cfg(feature = "runtime")] pub use self::http::{HttpConnector, HttpInfo};
 
 /// Connect to a destination, returning an IO transport.