Make client lifetime and shutdown more explicit in client example

[dekellum]

This is complimentary/in-addition to #1565. Originally I figured we might be able to resolve that one first, but then I found I needed to also reference this proposal.

Summary of examples/client.rs changes:

• Uses spawn, which is more async™ than run, and thus more educational for a real application.
• Avoids the magic of lazy(Fn) and moving the Client instance into a single request future. Thus this example is more amendable to multiple-requests on a shared Client for keep-alive, etc.
• Makes the shutdown sequence explicit and clarifies potential pitfalls.
• Adds a working default URL as a convenience, not unlike the doctest. HTTP-only websites are getting harder to find!

Note (as in #1565) tokio::runtime::Runtime isn't currently re-exported in hyper::rt. I could add it there instead, if desired (though I wonder about the sustainability of this re-export approach.)

The client sections of the guide also uses this example, so with some positive indication, I could pursue a compatible guide change PR.

[seanmonstar]

I think there is value in showing in the examples that you can just use run in many cases. Since the concern is around forgetting to put a Client in a lazy future and thus keeping the runtime from shutting down, what if there was just a comment added at that line about that?

[seanmonstar]

Thanks for the efforts to improve the documentation, I greatly appreciate it! I'm going to close this for now, and allow discussion to happen in #1571. If we find ways in that issue to improve things, new PRs can always be submitted.