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
Migrated from D5919 (view on Phabricator)