Row-store leaf-page key retrieval.

@michaelcahill, this is a set of changes to clean up key retrieval on row-store leaf pages.

In short, one of the underlying functions copied data for historical reasons and there was some undocumented magic in the paths to accessing row-store leaf pages.   I think I've teased it all apart, but it could use a review.   I don't think it's worth going deeply, but  flagging any comments or code changes that don't make sense to you would be welcome.

Thanks!