Investigate whether crossbeam types can help

In https://stjepang.github.io/2019/01/29/lock-free-rust-crossbeam-in-2019.html, @stjepang mentions a few pieces that may be relevant to `bus`. Specifically, [`Backoff`](https://docs.rs/crossbeam/0.7.1/crossbeam/utils/struct.Backoff.html) might be a good substitute for `parking_lot::SpinWait`, and [`CachePadded`](https://docs.rs/crossbeam/0.7.1/crossbeam/utils/struct.CachePadded.html) might buy us some better performance in certain cases.

Start with the linked 2019 article and the Crossbeam documentation for Backoff and CachePadded. Inspect the bus code for uses of parking_lot::SpinWait, then determine whether either Crossbeam type applies and where it could improve performance. Done means a documented recommendation with the relevant trade-offs.

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