Integrate Loom

[kvark]

See https://docs.rs/loom/latest/loom/
It's a magical tool to help debugging concurrency issues.
There is a small downside that this has to be a compile feature, and it's not useful to the outside (i.e. internal compile feature). But I think it's worth it anyway.

[kvark]

Latest blocker has been lack of support for Loom from crossbeam. I believe crossbeam is a bit of a heavy dependency, anyway, and it would make sense to implement a queue in-house. This would make crossbeam to be optional, and currently incompatible with loom, but at least we can test the whole thing without it.

[kvark]

Alternatively, we can use synqueue or rc_event_queue, both of which are friends with Loom already.