Fix several bugs associated with retry/sleep (#940)

* Fix several bugs associated with retry/sleep

The sleep implementation was late-bound but early loaded in retry which created a large surface area for bugs & meant that we emitted a lot of spurious warnings. This commit:
- Removes more kruft from aws-hyper and deals with the consequences
- Cleans up the way that HTTP clients are generated
- Passes in sleep to each incarnation of the retry controller to work around issues caused by late-binding vs. early binding sleep.

* first round of cleanups

* delete unused import

* Update integration tests

* Fix examples

* Fix aws-smithy-client docs

* Clippy fix

* fix flaky test

* Fix  which relies on tokio/io