pushdown power to TiFlash/TiKV

[windtalker]

Enhancement

TiDB already pushdown pow to TiFlash

tidb/pkg/expression/infer_pushdown.go

Line 277 in da501a0

ast.Sqrt, ast.Log, ast.Log2, ast.Log10, ast.Ln, ast.Exp, ast.Pow, ast.Sign,

And TiKV

tidb/pkg/expression/infer_pushdown.go

Line 185 in da501a0

ast.Ceil, ast.Ceiling, ast.Floor, ast.Sqrt, ast.Sign, ast.Ln, ast.Log, ast.Log2, ast.Log10, ast.Exp, ast.Pow,

According to MySQL's doc, power is just a synonym for pow, so it is ok to pushdown power to TiFlash/TiKV without extra work in TiFlash/TiKV side.