Replace reqwest with hyper (#8099)

* Very primitive test of the Client API

* [WIP] getting rid of request

* Add support for redirects.

* Remove CpuPool from `fetch::Client`.

* Adapt code to API changes and fix tests.

* Use reference counter to stop background thread.

On `clone` the counter is incremented, on `drop` decremented. Once 0 we
send `None` over the channel, expecting the background thread to end.

* Fix tests.

* Comment.

* Change expect messages.

* Use local test server for testing fetch client.

* Ensure max_size also in BodyReader.

* Replace `Condvar` with `sync_channel`.

* Re-export `url::Url` from `fetch` crate.

* Remove spaces.

* Use random ports in local test server.