samdb/schema_load: do schema loading with one search

samdb/schema_load: do schema loading with one search

It appears that there was a race condition between searching for the
attribute & class definitions, and searching for the schema object, if
the schema was changed in-between the two searches.

This is likely the cause of ldap_schema being flapping.

BUG: https://bugzilla.samba.org/show_bug.cgi?id=12889

Signed-off-by: Bob Campbell <bobcampbell@catalyst.net.nz>
Reviewed-by: Andrew Bartlett <abartlet@samba.org>
Reviewed-by: Garming Sam <garming@catalyst.net.nz>