Rename btree->{evict,root}_page -- they aren't WT_PAGEs any more.
It's okay if an LRU walk returns WT_NOTFOUND: that just means some page was locked.
Flip the logic around when to deepen the tree -- there is no point trying to deepen if there aren't enough children for the split.

Deepen sooner: when leaves grow and split in two repeatedly, it takes a lot of re-allocation for the parent to reach 200K children.
Make sure we always publish a required transaction ID before looking at page structures.  Otherwise, they can be freed from underneath us when a page splits.
Catch a case where snap_min is being cleared while cursors are open.  This turned out to be the metadata cursor open during a checkpoint.
Merge branch 'new-split' into new-split-fixes
Merge branch 'new-split' into new-split-fixes
Merge branch 'new-split' into new-split-fixes
Simplify pinning and releasing a transaction ID required to look at page index strutures.
Revert to asserting that a page is clean after `__wt_rec_write` returns zero, until we understand where that is coming from.
Merge pull request #943 from wiredtiger/new-split-fixes

Fix some bugs and races with new splits.