perf: replace `json.NewDecoder` with `json.Unmarshal` in the `om` package

[rueian]

As said in #864:

json.Decoder() copies data from the reader into a temporary buffer. This is an extra copy that is not necessary since we have the data as a slice already.

json.NewDecoder is not optimal for our use. We should replace the following with json.Unmarshal:

rueidis/om/json.go

Line 70 in 440c5e0

if err := json.NewDecoder(strings.NewReader(record)).Decode(&v); err != nil {

[KyleLin0927]

hello @rueian , I'd like to try it.

[rueian]

close by #867