FIX Trigger eagerloading for first() and last()

[GuySartorelli]

The new logic is directly pulled from SQLSelect::firstRow() and SQLSelect::lastRow() which is was ultimately was powering these methods - tracing those, you can see that this limited query (limit 1) was always being run, and the result (which was necessarily an array of size one) was returned from the database - and then just the first and last rows respectively were pulled out from there in PHP.

So the logic is unchanged, I'm just pulling it up to this level so we can also trigger eager loading - and only trigger it once. i.e. calling first() then last() and then iterating over the same list won't trigger multiple db queries.

Issue

• DataList doesn't trigger eagerloading for first() and last() calls #10876

[emteknetnz]

Unit tests should add 3x records each with different titles and then confirm that ->first() and ->last() have returned the correct objects

[GuySartorelli]

I had assumed DataListTest would already have that - but it doesn't! Which is worrying, but I'll add that now.