[18.0][ADD] account_credit_control_aggregate_level - Aggregate multiple levels in credit lines to make single communications

[yvaucher]

Forward port of #22

In case of recurring invoicing you might have multiple lines with different levels, however you don't want to multiply the communications to the creditor.

This adds the possibility to communicate only the highest due and attach the lower levels to it to not de-multiply the communications.

Compared to Apply max policy level, the aim is to keep distinct level values on credit lines and not create them at the same level.

"Auto Process Lower levels" means that you have to care only about the highest level and that other lines for the same partner, policy will be communicated as well.

[len-foss]

[ADD] is supposed to be for new modules. Here there's a small [REF] refactoring of the module, and mostly [IMP] improvements to it. Which I'm not sure should go into it and not into another module. The rationale for that is that this module is already quite buggy, so I would think keeping it as lightweight as possible would be best.

[yvaucher]

@len-foss Thanks for your input, I'll modify this PR to create a module account_credit_control_aggregate_level to extract this feature.

[len-foss]

@yvaucher that's great!
Since you're working on it, it would be good if you also have a check at #467
This PR mainly makes sure that it's the postprocess that update the line status based on whether the mail was really sent or not.

There's another that you're unlikely to catch with simple test:
#430
introduced a threading mechanism that makes that a good part of the emails are never sent because they're opened with their own cursor, they get a PoolError('The Connection Pool Is Full'), so the run is partly done. The lines have been marked as queued so without modifying data with a server action you have no way to recover from this state.
(so to test it, you need a number of lines that exceeds ODOO_MAX_HTTP_THREADS)
I'll push a PR this afternoon.