Update doc tests of client to async/await

[weihanglo]

What

Reference #1850

• Remove unnecessary extern crate.
• Remove unnecessary import such as futures::Future, hyper::rt::Future.
• Use async fn and await syntax instead of futures operators.

How to test

• Run cargo doc --open to check if examples are rendered properly.
• Run cargo test --doc -- src/client. It should output something like

running 10 tests
test src/client/conn.rs - client::conn::SendRequest<B>::send_request (line 193) ... ok
test src/client/connect/mod.rs - client::connect::Destination::set_port (line 172) ... ok
test src/client/connect/mod.rs - client::connect::Destination::set_host (line 131) ... ok
test src/client/connect/http.rs - client::connect::http::HttpInfo (line 48) ... ok
test src/client/mod.rs - client::Client<(), Body>::builder (line 140) ... ok
test src/client/mod.rs - client::Builder (line 824) ... ok
test src/client/connect/mod.rs - client::connect::Destination::set_scheme (line 104) ... ok
test src/client/mod.rs - client (line 29) ... ok
test src/client/mod.rs - client::Client<C, B>::get (line 178) ... ok
test src/client/mod.rs - client::Client<C, B>::request (line 208) ... ok