Disallow sub-classes of RelOptRuleOperand

Disallow sub-classes of RelOptRuleOperand. The only reason that people have sub-classed RelOptRuleOperand is to override the boolean matches(RelNode) method, and you can now achieve that by supplying a Predicate to the constructor.

Next release, we will deprecate RelOptRuleOperand's protected constructor and create a new private or package-protected one. Later we will remove that constructor.