Pods 2.5 optimizations work

This is a general PR for all the 2.x-optimizations work done so far by @jamesgol / @pglewis; to ensure we can see a full big picture of the changes. This will make it easier to review everything as it currently is (or becomes) and I can comment on the difference lines.
This was the heart of my WordPress caching arrays of objects issues.  If this gets unset here then it is removed from the objects in the cache and then they never get used because pod_item_id is unset