Allow to (only) store users in an external user database/backend

[freeaqingme]

Hi,

I'm busy on a project where I want to use an external user backend. Various solutions exist to mirror the user database to a remote service, but my goal was to have an entirely empty wp_users and wp_usermeta table. If my wordpress installation were to be compromised, this would provide an extra layer of security.

I was pleasantly surprised by the vast number of hooks that were already present to accommodate this, and only a few small modifications were necessary to also support my use case.

Please see the attached patch. Feedback is appreciated, of course.