Investigate why vBucket flush task appears to be scheduled behind other AuxIO tasks

As seen during MB-30501; the VBucketMemoryAndDiskDeletionTask had long scheduler wait times - where BackfillManager tasks seem to be getting scheduled ahead of it.

BackfillDiskLoad isn't actually the task which is involved in backfilling in this case - it's managed by BackfillManagerTask which is already a lower priority than VBucketMemoryAndDiskDeletionTask.
As such there is a more curious question why VBucketMemoryAndDiskDeletionTask exhibits such long wait times; given it should be the highest priority AuxIO task.