Tweak planner to use OFFSET+LIMIT, not just LIMIT, as estimate of the

Tweak planner to use OFFSET+LIMIT, not just LIMIT, as estimate of the
portion of the query result that will be retrieved.  As far as I could
tell, the consensus was that we should let the planner do the best it
can with a LIMIT query, and require the user to add ORDER BY if he
wants consistent results from different LIMIT values.