Split right

Not ready for merge.
@agorrod, @michaelcahill: I made a change in dd736a6 that does this -- we immediately return if there aren't at least two level-6 skip list entries (at 1/4 chance per level, I think that translates into roughly `(4^6) * 2) == 8,000` items on the list).

Feel free to revert that change if you don't like it, or tell me and I'll back it out.