Allow auth->getUserData to be retrieved with or without group memberships

Also implements caching of user data for mysql & pgsql plugins.

Resolves [FS#2086](https://bugs.dokuwiki.org/index.php?do=details&task_id=2086)
Addresses https://github.com/splitbrain/dokuwiki/issues/587
You're right there is an error here.  On the face of it, _getUserInfo() would appear to be the correct function call. I'm concerned as to why I changed it.  I guess, it could be some unreverted code change mis-committed.  I'll look into it.