recurrent_visits: Allow to set no origins scheduled backoff in config
The send_visits_for_visit_type function uses a default schedule backoff of 20 minutes where there is no origins to schedule for a given visit type.
It exists use cases when we would like that schedule backoff to be shorter in order to schedule listed origins for loading into the archive more rapidly, typically in the docker environment.
So allow to set that backoff value through configuration.
The purpose of that diff is to schedule loading tasks for listed
origins in the docker environment without having to restart the
swh-scheduler-schedule-recurrent
service.
Test Plan
I did what I could to test this but things are hard to mock here as it involves threading.
Migrated from D8475 (view on Phabricator)