Fix in-memory splits if the last item in the skiplist is also the deepest.
refs #658
Further fix and cleanup of the in-memory split change.
Fix in-memory page split skip list.

When traversing the skip list, we (normally) start at the depth of the item
we are removing, not depth + 1.
Rewrite the main loop to always start from the head of the skiplist and deal with corner cases.
Fix in memory page split logic when updating skip list.
Merge pull request #658 from wiredtiger/inmem-split-fix

Fix some corner cases in splitting a skip list.