Improve speed, especially with large data sets

_globalItems is not a cache of the index paths (at least, in UICollectionView it isn't). This commit removes that concept for now. Eventually, it should be added back in, but when it does there are a whole load of other methods that need adding such as 'loadEverything' and
'setLayoutAttributes:atGlobalItemIndex:'. We should do that, but that's for another day.

I found all this out from using Hopper on UIKit. I think PSTCollectionViewData should have some love given to it to potentially improve speed even more. We're currently using PST with an incredibly large data set, so we'll continue to work on it and push things back as we find them. Hopefully I'll get time to add _globalItems back in and follow the rabbit hole that I opened up when looking into this.