Provide a unified API for listers to interact with the scheduler

Currently, listers interact with the scheduler to create recurrent loading tasks, and pull back references to tasks into their own database schema.

In order to improve our scheduling of recurrent visits, we need listers to provide more accurate and uniform data to the scheduler, rather than just plain loading tasks.

We should also keep in mind how such a unified API would help us to provide the data backing a full implementation of swh-web#1870 (closed).

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Migrated from T2442 (view on Phabricator)