Start handling of recurrent loading tasks in scheduler

• This deals with the next_position_offset update done by the scheduler journal client.
• Introduce storage endpoint for the recurrent visit scheduler queue position read/write.

TODO:

• update the journal client so it computes the queue position per visit type (!324 (closed))

Related to #2345

Test Plan

tox

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Migrated from D5919 (view on Phabricator)