ENH Cache $item->ID for eager loading

[emteknetnz]

Issue silverstripe/silverstripe-elemental#1067

This small change makes a remarkably large difference to eager loading performance.

Every call to $item->ID in the loop will call ViewableData::__get(), which in turns ends up creating a new object via Injector. While this is a fast operation, if you have a huge number of calls, such as in my case where I was working on the highly unoptimized cms site search, with 1000 pages and 5000 content blocks, this saved over a million calls per search. All this PHP object creation was so bad it made enabling eager loading actually slower despite the drop in database calls.

This change makes it so that eager loading now faster in this particular operation, as would be expected

[GuySartorelli]

There's probably a lot of optimisation that can be done for this feature.... but in any case this PR has a very narrow scope, and I have seen first hand that this is a very clear improvement in itself, so I'm fine merging this on its own merits.