Instead of fixing up the 0th key on split internal pages during the
final merge, maintain a correct key on every split page we created.
Reference #457.
Merge branch 'develop' into merge.457
Merge branch 'develop' into merge.457
Remove the in-memory merge code that tried to deal with missing 0th keys.
Merge pull request #465 from wiredtiger/merge.457

Internal split page 0th key changes.