Don't split pages during either pass of checkpoint because the act of
splitting pages into the parent creates pages with no address, and
checkpoint of the internal page requires leaf pages to have addresses
(with the single exception of "deleted then re-instantated pages).
Merge branch 'new-split-overflow' into new-split-ckpt-1
Merge branch 'new-split-overflow' into new-split-ckpt-1
Merge branch 'new-split-overflow' into new-split-ckpt-1
Revert the changes to date: we're going with a simpler solution.
During the final phase of checkpoint, visit all pages: otherwise, we could split a leaf page without writing it, then fail to write the internal node.

refs #913
Merge pull request #914 from wiredtiger/new-split-ckpt-1

checkpoint can race with splits