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

Now that the fast-append search code is inlined, we shouldn't need to
optmize out the check that an insert list exists.
Merge branch 'row-search-slim' into row-search-slim-fast-append-first
Merge pull request #1122 from wiredtiger/row-search-slim-fast-append-first

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