JS function XWiki.Document#getURL does not take into account the wiki name

I prefer an async URL factory API that requests the needed wiki URL template on demand, and caches it. I wouldn't preload the URL templates for all the available wikis because there can be a lot of them, and we rarely need all, if ever. I'm not sure what's the best name for this API though. I'm not too fond of getURLAsync but it can work.