Optimize sub-queries for <, <=, >, >=, =, <>

[janetzki]

The LQP optimizer should use a join for queries like

SELECT *
FROM customer
WHERE c_acctbal < (
  SELECT MIN(o_totalprice)
  FROM orders
  WHERE o_custkey = c_custkey
)

(Returns all customers who cannot afford any of their orders.)

• Implement this feature on a separate branch