voyager cleanup and some docs (#1640)

- changed the `Msg` variant in `queue-msg` to `Effect`, as this is more
accurate and generic
- made naming of `QueueMsgTypes` implementors more consistent
- slightly adjusted the docs for voyager, will continue editing this in
followup prs
- renamed `queue_msg::Reactor` to `Engine`
- added `Engine::step`, which will be used in testing
- in doing so, was able to refactor `Engine::run` to use lifetimes
instead of cloning