Scheduler runner is slow to run tasks

The scheduler runner is slow to pick out tasks that need to be scheduled. This is most obvious on the origin metadata indexer tasks, where workers manage several dozen tasks per seconds, and the bottleneck is the buffer.

This is probably, in part, an issue with the schema used on the database, be that the schema itself or indexes. We should investigate (measure) and fix that.

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Migrated from T1547 (view on Phabricator)