Mark ScheduleAggregationWorker as idempotent

What does this MR do?

This marks the Namespaces::ScheduleAggregationWorker as idempotent. It was already idempotent, because it checks the existence of the Namespace::AggregationSchedule before doing anything and it uses safe_find_or_create to avoid race conditions.

This adds specs proving that it is idempotent.

It also adds the worker to the DROPPABLE_QUEUES so we can use it to try out deduplicating jobs.

Needed for: gitlab-com/gl-infra/scalability#218 (closed)

Does this MR meet the acceptance criteria?

Conformity

• [-] Changelog entry
• [-] Documentation (if required)
• Code review guidelines
• Merge request performance guidelines
• Style guides
• Database guides
• Separation of EE specific content

Availability and Testing

• Review and add/update tests for this feature/bug. Consider all test levels. See the Test Planning Process.
• [-] Tested in all supported browsers
• [-] Informed Infrastructure department of a default or new setting change, if applicable per definition of done