Multiple procrastinate instances on the same DB: namespaces

[ewjoachim]

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We have the case where several projects sharing the same DB want to use procrastinate independently.

We're going to support namespaces. A nullable namespace column will be added to procrastinate_jobs. Each function will take namespace as an optional parameter, and restrict its results on namespaces. LISTEN queue names will contain the namespace.

Namespaces will be provided to the App by a constructor kwarg, and passed to the Connector that will inject it in every query.

[ewjoachim]

ping @k4nar FYI :)