iris: Poor performance of session.UpdateExpiration on 200 thousands+ keys with new radix lib

Hello Makis!

I have more 200 thousands key in redis on prod server and found that after upgrading iris on v11 from redigo to radix lib that I have problems with perfomance when calling session.UpdateExpiration method (it`s executing minimum 10 seconds on a powerful prod server and more than 30 seconds on usual local).

While debugging I found place where freezes, it`s here:

func (r *Service) UpdateTTLMany(prefix string, newSecondsLifeTime int64) error {
	keys, err := r.getKeys(prefix)
	if err != nil {
		return err
	}
...
}

and inside getKeys here:

scanner := radix.NewScanner(r.pool, radix.ScanOpts{
		Command: "SCAN",
		Pattern: r.Config.Prefix + prefix + r.Config.Delim + "*", // get all of this session except its root sid.
		//	Count: 9999999999,
	})

	var key string
	for scanner.Next(&key) {
		keys = append(keys, key)
	}

Just has watched old v10 iris and old method getKeysConn did not cause such delay …

func (r *Service) getKeysConn(c redis.Conn, prefix string) ([]string, error) {
	if err := c.Send("SCAN", 0, "MATCH", r.Config.Prefix+prefix+"*", "COUNT", 9999999999); err != nil {
		return nil, err
	}

	if err := c.Flush(); err != nil {
		return nil, err
	}

	reply, err := c.Receive()
	if err != nil || reply == nil {
		return nil, err
	}

Maybe reason in difference between old and new scan commands? Could you help?