erp5_core: reindex Balance Transactions in order

I made a custom script while doing Nexedi ERP5 reindex, but never took the time to commit nor upstream anything, here it is. I do not want to change it too much now, since it worked that way if I remember correctly, but I think at some point one might want to make a general script that works with all objects to be reindexed in order (Inventories, Holiday Acquisitions, Balance Transactions, etc).

/cc @jerome