Split right

Not ready for merge.
We do know if we're inserting anywhere on the page that an insert-split won't help (anywhere but that last insert list on the page). I didn't do the actual work because it's a shared cache line that I'm not excited about touching, and a single insert anywhere on the page would turn off insert-splits which isn't right either.

I'll delete the comment, it's not a good idea.

What would you think of checking that the insert list we're splitting is actually large enough to split? Get some kind of quick estimate of how many elements it has?