Modify queue usage to be more self-contained

Similar to #5525, or figure out if that doesn't work: where and why.

Basically:

* Keep queues private to a class or at most shared among a few related files (let's try not to). Don't pass them around to other types
* Default to main
* Always operate on main across boundaries

So might be useful to search for ", queue: ", "(queue: " and ", queue: \w*)"