DocumentUserManager should use a cache

We introduced in 12.2 a UserManager whose only API for now is to check existence of a user.
Right now this check can be a bit expensive since it implies to load the XWiki Document to check if there's the right class in it.
It would be an easy perf improvment to add a cache at least in DocumentUserManager that would contain the information if a reference is an existing user or not. A cache entry would be invalidated whenever the reference is updated / created / deleted.