r5467: Optimize _samr_query_groupmem with LDAP backend for large domains.

r5467: Optimize _samr_query_groupmem with LDAP backend for large domains.

Could someone else please look at this patch, verifying that I did not break
the ldapsam:trusted = False fallback to the old behaviour? It works fine for
me, but you never know. You're certainly free to review the new code as well :-)

Thanks,

Volker
(This used to be commit e1c3ca182b299dc65da1fa39aadb69876b5e16b8)