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
_getCachedUserInfo() is missing. Is this the wrong function name maybe?