Item_sum_variance: move recurrence_m, recurrence_s, count to a separate class

The class Item_sum_variance has the following members:

  double recurrence_m, recurrence_s;    /* Used in recurrence relation. */

  ulonglong count;

We're going to change recurrence_m and recurrence_s from double to long double, to improve the result precision.

It makes sense to move these three members into a separate class Stddev and encapsulate the exact internal representation into this class.