Simplify the row-store internal search loop

@michaelcahill, this is the change to pull the comparison against an index of 0 out of the internal page binary search loop.

It looks to me like this change simplifies things, including tests after the loop exits.

I don't think there's any way we can get out of the loop without either an exact match or base set to `(correct-slot + 1)`, but I'd like it if someone else stared at this carefully.