When generating an empty in-memory page to satisfy a search into a deleted key
range, we were instantiating a modify structure and mark the new page with
WT_PM_REC_EMPTY so a concurrent checkpoint will skip over it (commit
989e2ffbcfe66b096199729cdac7bddc9d8d3aa0).   This commits handles that case in
a different way, by detecting such pages when reviewing the children of an
internal page.  Reference #437.
    Fix the problem described in issue #345 where we could potentially leak
pages if we quit early when evicting an internal page where all the changes
weren't visible; change reconciliation of internal pages to quit early if
evicting an internal page where all the changes weren't visible -- this is
more of a cleanup than a bug fix, but we don't want to have any confusion
here.
    Rework the row-store internal page code that handles underlying page
modifications: return an explicit value if the child is modified, needs
a deleted-page proxy cell, or should be ignored.
Code shuffling with no real change: move the primary function
__wt_rec_write to the top of the file.
Merge pull request #439 from wiredtiger/evict-empty-2

Deal with empty child pages when reconciling internal pages.