Add indexes on account payment models

The fields where the indexes are added are used in searches in
account_payment_order, which becomes really slow when a database have
many lines.

account_payment_order/models/account_move_line.py

@@ -15,7 +15,9 @@ class AccountMoveLine(models.Model):
         help='Bank account on which we should pay the supplier')
     bank_payment_line_id = fields.Many2one(
         'bank.payment.line', string='Bank Payment Line',
-        readonly=True)
+        readonly=True,
+        index=True,
+    )
     payment_line_ids = fields.One2many(
         comodel_name='account.payment.line',
         inverse_name='move_line_id',