samba-tool domain schemaupgrade: Avoid reindex after every hunk

samba-tool domain schemaupgrade: Avoid reindex after every hunk

This takes advantage of the fact that a single LDB operation is atomic
even inside our transaction and so we can retry it after updating the
schema.

This makes the smaba-tool domain schemaupgrade take 1m30s compared with 4m4s.

Signed-off-by: Andrew Bartlett <abartlet@samba.org>
Reviewed-by: Garming Sam <garming@catalyst.net.nz>
Autobuild-User(master): Garming Sam <garming@samba.org>
Autobuild-Date(master): Thu Dec 21 08:28:51 CET 2017 on sn-devel-144