[*] BO : performance improvement on AdminInvoices loading time

On a shop with lots of orders, this page takes forever to load. Why ? Because this request is really slow, just to show "how many orders are with this status".

Datas :
On a shop (1.5.6.0) with 150 000 orders
Request used to take up to ~50s
Time cut down to 5s for the same results on the same page after optimization