[IMP] stock: merge moves

The rationale for the need of a merge move mechanism is that, with the new default view,
having multiple stock move for the same products is confusing. The idea is that if a move
is confirmed in a picking already having a move with the same characteristics, we update
the existing move and unlink the new one. That’s why it’s done at the end of `_action_confirm`.

In some cases, merging moves is unwanted (backordered moves for examples). Therefore, we chose
to add a kwarg to `_action_confirm` in order do control the mechanism.