func RandIDWithLen(idLen int) (id string, err error) {
	b := make([]byte, idLen)
	_, err = rand.Read(b)
	if err != nil {
		return
	}
	id = fmt.Sprintf("%x", b)
	return
}
RandIDWithLen return a rand string with idLen length.
package util
import (
	"crypto/md5"
	"crypto/rand"
	"encoding/hex"
	"fmt"
	"strconv"
	"strings"
)
func RandID() (id string, err error) {
	return RandIDWithLen(8)
}