support for custom DNS resolution

[irrelevelephant]

This change refactors DNS resolution, with the goal of supporting custom DNS resolvers. It addresses #1125.

Previously, reqwest defined its own HttpConnector, which dispatched to a different hyper::client::HttpConnector<R> instance depending on an enum state. This led to excess complexity in the Future type associated with the Service<Uri> implementation for the connector, which can be circumvented with a trait object.

This approach exchanges the enum for a trait object, which introduces a dynamic dispatch, but eliminates the enum match inside HttpConnector.

One annoyance is that trait objects can't make use of associated types, so the return type of the resolve function on the trait needs to be explicitly defined. Let me know what you think - happy to make changes.

[seanmonstar]

This is some awesome stuff. Just a few discussions I started inline.

[niuhuan]

Can you give me an example?

[irrelevelephant]

@niuhuan You just need to implement the Resolve trait, then pass a trait object to the client configuration via the dns_resolver method.

A simple example of implementing the trait is the GaiResolver.

impl Resolve for GaiResolver {
    fn resolve(&self, name: Name) -> Resolving {
        let this = &mut self.0.clone();
        Box::pin(Service::<Name>::call(this, name).map(|result| {
            result
                .map(|addrs| -> Addrs { Box::new(addrs) })
                .map_err(|err| -> BoxError { Box::new(err) })
        }))
    }