ldb: Allow a caller (in particular Samba) to handle the list of attributes with an...

ldb: Allow a caller (in particular Samba) to handle the list of attributes with an index

By doing that, Samba will use a binary search to locate the attributes
rather than an O(n) search, during every search or modify of the database.

Signed-off-by: Andrew Bartlett <abartlet@samba.org>
Reviewed-by: Stefan Metzmacher <metze@samba.org>
Reviewed-by: Garming Sam <garming@catalyst.net.nz>