Optimize StorageJanitor::findOrphanedItems

Carl Schwanrequested to merge
work/carl/optimize-query-storagejanitor into master

Use subqueries to optimize the find orphaned items.

Alternative to !298 (closed) faster but doesn't use the SqlBuidlder as this doesn't support subqueries

BUG: 451620 FIXED-IN: 6.6.0

Edited by Carl Schwan

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