parser/auth: caching_sha2 can be faster

```
goos: darwin
goarch: amd64
pkg: github.com/pingcap/tidb/parser/auth
cpu: Intel(R) Core(TM) i5-8257U CPU @ 1.40GHz
BenchmarkShaPassword
BenchmarkShaPassword-8   	     382	   3101911 ns/op	  320674 B/op	   10022 allocs/op
```

it will allocate lots of objects at now