Change `Resolve` to alias `Service`

[seanmonstar]

After #1782 is done, the Resolve trait should be changed to basically an alias for a Service<Name>. This would allow more composability, being able to use ServiceExt to customize resolvers.

[bstrie]

Now that #1792 is closed, what do you have in mind by "basically an alias"? I assume this isn't referring to waiting for rust-lang/rust#41517 , so what should Resolve become?

[seanmonstar]

It'd be similar to HttpService, basically just bounds that mean R: Service<Name, Response = impl Iterator<Item = IpAddr>>. As the top comment mentions, that'd allow composing Services, such as easily adding in a timeout layer.