Investigate whether crossbeam types can help

[jonhoo]

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 might be a good substitute for parking_lot::SpinWait, and CachePadded might buy us some better performance in certain cases.

[jonhoo]

Parker may also be a good thing to use instead of our thread::Thread.