backport of #11494

fix retrievel of group members and cache group members

fix changed variable name

with several backends, more than limit can be returned

make performance less bad. Still far from good, but at least it works

add one simple cache test

adjust group manager tests

Conflicts:
	apps/user_ldap/group_ldap.php
	apps/user_ldap/lib/access.php
	apps/user_ldap/tests/group_ldap.php