ItemPager/ExpiryPager/PagingVisitor refactor

A number of improvements could be made to these classes.

• Consider creating two ItemPager tasks, one explicitly is triggered by an interval, the other is triggered by 'out-of-memory' conditions.
• Consider structuring the tasks so that the ItemPager doesn't just end up scheduling a second task to do the visiting, consider integrating the visiting into the run loop (removing the shared_ptr<atomic_bool> available stuff).
• Consider structuring the PagingVisitor to remove the need for the owner, perhaps EvictionVisitor and PagingVisitor which share a common parent class.
• Likely more improvements too.