LDAP: reset user/group-config association only after exists-check, may p...

...erformance in some cases

Explanation: because we support multiple LDAP servers, we have a proxy that delegates a method call to the correct backend. To avoid roundtrips, we cache to which LDAP server a user belongs. Sometimes we reset this on a per-user basis. Until now when the method returned false. However, "false" can be a very valid return value. So we check whether the user (or the group) still exists on the LDAP server before resolving the association.

In some cases it improves performance noticeable.

@karlitschek @bartv2 @MTGap or others, thanks.