remove log prints or enable log level setting

[sasakiyori]

Every time one instance update a policy, it will print:

Get revision:  1
Set revision:  2

I actually don't need these prints, and these prints come from log.Println(), it is hard for me to change from outside.

Can these logs be removed? I think such logs are meaningless in a base package: If I want to know if the Update function work normally, I can use debug tools. If I want to know the revision of etcd key, I can straightly get it by etcdctl or some how.

Or enable a log level setting, like casbin/log.Logger do. This also help for me.

Code From:

func (w *Watcher) Update() error {
	rev := 0
	resp, err := w.client.Get(context.Background(), w.keyName)
	if err != nil {
		if err != rpctypes.ErrKeyNotFound {
			return err
		}
	} else {
		if resp.Count != 0 {
			rev, err = strconv.Atoi(string(resp.Kvs[0].Value))
			if err != nil {
				return err
			}
			log.Println("Get revision: ", rev)
			rev += 1
		}
	}

	newRev := strconv.Itoa(rev)

	log.Println("Set revision: ", newRev)
	_, err = w.client.Put(context.TODO(), w.keyName, newRev)
	return err
}

[casbin-bot]

@tangyang9464 @JalinWang @imp2002

[Yang Luo]

@sasakiyori can you make a PR to fix it?

[sasakiyori]

@hsluoyz sure, I'd like to work on this. BTW I prefer to remove these log prints straightly for the reasons mentioned above.