python join: Set index transaction cache size.

python join: Set index transaction cache size.

The default value is too small for joining a large domain.  So we specify a
size of 200,000 which is suitable for domains with up to 100,000 users.

At a later date this could be added as a parameter to the join, but
200,000 should be suitable for now.

Signed-off-by: Gary Lockyer <gary@catalyst.net.nz>
Reviewed-by: Andrew Bartlett <abartlet@samba.org>
Autobuild-User(master): Andrew Bartlett <abartlet@samba.org>
Autobuild-Date(master): Thu Apr  4 07:45:03 UTC 2019 on sn-devel-144