[10.0][IMP] Set invoices paid

[StefanRijnhart]

In Odoo 9.0, an invoice was moved from state 'open' to state 'paid'
on reconciliation of the move line that was linked as the trigger of
payment subflow in the invoice workflow. This flow usually prevented
zero amount invoices from getting into state paid because zero amount
move lines are usually not reconciled. The invoice would be marked as
reconciled correctly, because there was no residual amount.
In Odoo 10.0, any invoice that is marked as reconciled will be moved to
state paid as per action_invoice_paid that is triggered from the
model's _write method¹. Preemtively set the correct state using SQL here
for performance reasons.

¹ https://github.com/odoo/odoo/blob/10.0/addons/account/models/account_invoice.py#L365

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[MiquelRForgeFlow]

LGTM

[OCA-git-bot]

This PR has the approved label and has been created more than 5 days ago. It should therefore be ready to merge by a maintainer (or a PSC member if the concerned addon has no declared maintainer). 🤖