Deduplicate Namespaces::ScheduleAggregationWorker as a PoC for job deduplication

During the last 7 days, we performed about 900.000 duplicate jobs for the Namespaces::ScheduleAggregationWorker.

This worker gets scheduled in several cases through the UpdateProjectStatistics concern, in total we've seen these callers:

json.meta.caller_id.keyword: Descending	Count	Sum of json.duration	Average json.duration
ProjectImportScheduleWorker	57,528,644	46,921,993.56	1.632
PostReceive	76,136,085	21,501,246.07	0.565
UpdateAllMirrorsWorker	57,774,717	20,301,395.10	0.703
BuildFinishedWorker	52,909,823	6,103,576.09	0.231
PipelineProcessWorker	60,419,877	5,540,395.56	0.183
/api/:version/jobs/:id	61,731,197	4,308,231.52	0.14
/api/:version/jobs/request	49,571,565	2,189,096.06	0.088
Repositories::GitHttpController#git_upload_pack	76,976,916	1,793,317.51	0.047
PipelineUpdateWorker	18,106,869	1,217,426.92	0.134
RepositoryUpdateMirrorWorker	30,791,186	1,095,830.15	0.071
https://log.gprd.gitlab.net/goto/b5adba783edac35da929be89e3889d83

This worker creates a Namespaces::AggregationSchedule record to be consumed by the Namespaces::RootStatisticsWorker.

To do this, we need to mark the worker as idempotent, and add it to the list of deduplicatable workers.

We can easily see something goes wrong: in the worst case we won't see any Namespaces::RootStatisticsWorkers be scheduled anymore. But no data is lost, we just need to start creating the records again, since that will cause the Namespaces::RootStatisticsWorker to be scheduled again.

This is a test of #42 (closed) on a queue that has less impact than enabling on all queues in one go.