[16.0][ADD] sql_export_delta

[hbrunn]

This module extends the functionality of sql_export to allow you to
only export changes between export runs.

I added a second commit that allows users to keep the results of an sql export, that's useful independently of deltas, and very useful with deltas.

[OCA-git-bot]

Hi @legalsylvain,
some modules you are maintaining are being modified, check this out!