Improve the performance of sum(int) in TiFlash

[windtalker]

Enhancement

Currently, in TiFlash the result type of sum(int) is bigint, and the result type of sum(decimal) is decimal, in TiDB/MySQL, sum(int) will return decimal. In order to make TiFlash compatible with TiDB/MySQL, the planner in TiDB add an implict cast of sum(int) when pushdown to TiFlash like
sum(cast(int as decimal)).(Note we can not add cast after sum like cast(sum(int) as decimal) because sum(int) may overflow.)

This brings an extra cost when calculating sum(int) in TiFlash, which may hurt the performance a little bit. Ideally, we should change the behavior of sum(int) in TiFLash so it can return decimal directly.