introduce monitoring status variables for max_recursive_iterations

max_recursive_iterations limits the amount of iterations, but currently it is not possible
to monitor the current amount of iteration to estimate a reasonable limit.

I suggest to introduce 3 new status variables on global and session base.

recursive_iterations_avg

1. average amount of iterations of all recursive cte statements.

recursive_iterations_max

1. highest amount of iterations of all recursive cte statements.

recursive_iterations_reached_max

1. how often the max_recursive_iterations limit was reached