Reduce computation complexity of group_concat()

As of 1.5 group_concat tries to precisely calculate the length of a result string elements. This is waistfull thus we should employ an heuristic,e.g. all of the elements takes 75% of the maximum length for the datatype.
We can compare the resulting string length with the allowed maximum length in ::getResult(). The refactoring is partially done(for DECIMALS) in MCOL-641.
Given this approach the estimate should be also calculated only once given we know RowGroup size.