Fix double spill.

This fixes an issue where split nodes can be double spilled. This is typically
not noticable but it can have large effects when bulk inserting as double
spilled nodes will get added to the freelist which will grow quickly.
Merge pull request #227 from benbjohnson/fix-double-spill

Fix double spill
Fix root split on very large append.

This commit fixes a bug where a root split on a very large insert would
cause an overflow of the root node. The problem was that the new root
was not split when it was created so a new root with more than 64K child
nodes would overflow the page.count (uint16).
Merge pull request #228 from benbjohnson/fix-large-append

Fix root split on very large append.