Split right

Changes to split the large insert lists that result from append-only workloads.
I should have thought more carefully about your question. It makes sense to do the in-memory split even if there is a checkpoint going on. I've pushed a change to move the test up where you suggested.

The test for not doing this if we aren't the first time into __rec_review is because that means we are evicting an internal page - and we want to do the actual eviction in that case.