Fix the SAMR cache so it works across completely insane

Fix the SAMR cache so it works across completely insane
client behaviour (ie.:
open pipe/open SAMR handle/enumerate 0 - 1024
close SAMR handle, close pipe.
open pipe/open SAMR handle/enumerate 1024 - 2048...
close SAMR handle, close pipe.
And on ad-nausium. Amazing.... probably object-oriented
client side programming in action yet again.
This change should *massively* improve performance when
enumerating users from an LDAP database.
Jeremy.

git-svn-id: svn+ssh://svn.samba.org/data/svn/samba/branches/SAMBA_3_0@11793 0c0555d6-39d7-0310-84fc-f1cc0bd64818