Want to have 1 command "shouldSplitAndMedian??" that figures out if we should split, and if so the split point.
While doing, also want to change way we determine if we should split.

Instead of actually counting data, just want to walk index and assume each object is the average object size.
Will make much faster, and also not require all the data to fit in ram.

Also - should make it yield as well, just in case it has to page in index.