Do the fast-append test before we search the leaf page.

@michaelcahill, one more "interesting" change.

Once the fast-append code is factored out of the general insert-list search function, we can do the fast-append check before doing the leaf page binary search.

I made this a separate branch because it implies an additional comparison in the case where we end up in the last page of the tree, but we're not appending, and a lot fewer comparisons in the case of appending a bunch of data to the tree, and I thought that deserved some more thought.   (It's only the last page because the internal page binary search loop will turn off the appending flag if we're not at the end of the tree.)