* expression: fix return type and decimal len for builtin aggregation function 'avg' and 'sum'
* Code format refine
* Code format refine
* Fix frac for avg
Aggregation results were stored in aggregation functions, so one map is created for each function to store different groups’ results, which requires much memory.
The aggregation functions are refactored and are now stateless, and the maps are kept in executor (executor.HashAggExec and mocktikv.aggregateExec) so that only one map is needed to store all the functions’ results, which reduced the memory usage.
