func isKeyOutbound(key string) bool {
	return key[:2] == outboundPrefix
}
Return true if key prefix is outbound
package mqtt
import (
	"fmt"
	"strconv"
	"github.com/eclipse/paho.mqtt.golang/packets"
)
const (
	inboundPrefix  = "i."
	outboundPrefix = "o."
)
type Store interface {
	Open()
	Put(key string, message packets.ControlPacket)
	Get(key string) packets.ControlPacket
	All() []string
	Del(key string)
	Close()
	Reset()
}
func mIDFromKey(key string) uint16 {
	s := key[2:]
	i, err := strconv.Atoi(s)
	chkerr(err)
	return uint16(i)
}