Fix merge-split regression.

This commit reverts merge-split and fixes the node.split() to do a multi-page split. This issue
caused problems with bulk loading because it would split into a small page and a very large page.
The very large page, in turn, would be an arbitrary size so when it was freed later it would be
difficult to reuse and would cause serious fragmentation issues.
Merge pull request #202 from benbjohnson/refactor-split

Fix split-merge regression.