Reduce the number of log for active push

## Feature Request

### Describe your feature request related problem
```
$ cat /path/to/pd.log | grep "2021/03/31 17:01.*send.*active" | wc -l
1555
```

### Describe the feature you'd like

We should reduce the number of active push operator logs.

### Describe alternatives you've considered

Perhaps we can change the log level or use another file to record