Avoid second pass over row-store leaf pages when they're read.

@michaelcahill, @sue: this change avoids the second pass when reading in row-store leaf pages, as long as there aren't any zero-length values on the page.

Michael: we could do a bit better and avoid the second pass in all cases, but I think it would cost us 4B in the page header, I don't see any way to do it without growing that structure.  Let me know what you think, if we should just grow that structure.