tor-proto: Introduce type aliases for some queues

This will make it easier to change their types.

This is hopefully not very controversial (or, indeed, interesting).

It's textually large, so I'd appreciate a quick review before it rots.

In my own branch, I change the types of these aliases, which demonstrates that I found all of the relevant occurrences. If that should turn not to change on main, we'll detect that later. So right now I don't think we need to worry about that possibility.

Part of #351.