ldb: Intersect the index from SCOPE_ONELEVEL with the index for the search expression

ldb: Intersect the index from SCOPE_ONELEVEL with the index for the search expression

This helps ensure we do not have to scan all objects at this level
which could be very many (one per DNS zone entry).

However, due to the O(n*m) behaviour in list_intersect() for ldb
1.2 and earlier, we only do this for small numbers of matches on the
filter tree.

This behaviour will only be for ldb 1.2 and will not be kept
long-term in LDB, versions 1.3.1 and above will instead only
intersect when the GUID index is in use.

NOTE WELL: the behaviour of disallowDNFilter is enforced
in the index code, so this fixes SCOPE_ONELEVEL to also
honour disallowDNFilter if there @IDXONE is enabled.  Again,
this will change again in 1.3.1 and above.

BUG: https://bugzilla.samba.org/show_bug.cgi?id=13191

Signed-off-by: Andrew Bartlett <abartlet@samba.org>
Reviewed-by: Garming Sam <garming@catalyst.net.nz>
(adapted from ef240aaca0ef693a96726ac2366c454294b87b96 in master)