A problem in the new-split branch is WT_REF structures remain with the
internal parent page, even when the tree deepens, and the memory
footprint of that page continues to increase even as the tree deepens.
The current algorithms go south: they view a large internal page as a
reason to deepen the tree, and no good comes the internal page never
decreasing in size.

A different approach: allocate WT_REF structures individually (even when
reading pages from disk), and move the memory footprint for the WT_REF
structures to the new children pages when the tree is deepened.
It's not really a "parent ref" any more, rename "pref" to "ref".
simplify/document error handling/cleanup when building in-memory
pages in service of eviction.
Merge branch 'new-split' into new-split-ref-allocate
Merge pull request #930 from wiredtiger/new-split-ref-allocate

Individually allocate WT_REF structures and move them when deepening the tree.