Deduplicate ProjectImportSchedule until it's executed
What does this MR do and why?
Multiple duplicated workers cause issues with locks: Multiple duplicate ProjectImportScheduleWorker ... (#390729 - closed)
It makes sense to deduplicate the worker until it's executed and reschedule one just after it's finished in case deduplication happened once: https://docs.gitlab.com/ee/development/sidekiq/idempotent_jobs.html#until-executed