Because restartless addons are updated while the application is running, they can potentially be updated while the user is actively using that addon, or when the addon is doing a task that the user wouldn't want interrupted. So we should support some API to ask restartless addons whether it's safe to update or not. It may or may not be worth also supporting a time estimate, kinda like "ask me again in 5 minutes".
Maybe send the add-on a callback that it should call when it's ready to be updated? If it never calls it then we update on restart or ask it again during the next update check?