Before, avg(col) will use sum(col)/count(1) to calculate.
After including count column aggregation
KYLIN-3883, avg(col) will use sum(col)/count(col) to calculate.
If there is no predefined count(col) measure, query with avg(col) will get exception "NoRealizationFoundException: No realization found for OLAPContext", which will effect the query on old cubes. So we should consider compatibility.