Don't call the insert list search routine unless there's an insert list
to search, it's a wasted function call in a common path.
Inline the fast-path insert-list append code and only call it when doing
a tree search, not when fixing up a cursor movement.

Rework the "appending" key so it only applies when appending to the page
as opposed to appending to any insert list (hopefully avoiding comparisons
both on leaf and internal pages where the comparison is unlikely to succeed).
Add a comment explaining the WT_BTREE.appending behavior.