Remove high-level pooling Client

[seanmonstar]

The hyper::Client and support modules hyper::client::connect should be removed from hyper, and parts should reappear in hyper-util.

• Move the Pool to hyper-util #2860
• Move the hyper client connect stuff to hyper-util #2859

[cameronelliott]

@seanmonstar I was going to submit a one line PR to the docs of hyper::client::Client
along the lines of "Client supports concurrent requests to multiple different host names".
(I wrote a simple test harness & used two different slow http servers to verify how this worked)

But I just discovered this PR+issue. (substantial refactoring)

For me, it was hinted, but not clear that I could use Client concurrently for different hostnames.
I really think the docs would benefit from an explicit statement regarding this.
Let me know if I can help on that somehow.