Enumeration is non-deterministic

[juripetersen]

Using Wayangs default enumeration and pruning strategies, the plans compared in cost models are non-deterministic.
This can be observed when comparing costs with different mechanisms:

            final PlanImplementation bestPlanImplementation = executionPlans.stream()
                .reduce((p1, p2) -> {
                    final double t1 = p1.getSquashedCostEstimate();
                    final double t2 = p2.getSquashedCostEstimate();
                    return t1 < t2 ? p1 : p2;
                })