fix: uniqMcs use all cpu

[ningmingxiao]

I find len(mcsList) == int(catRange*(catRange-1)/2) will cause uniqMcs use all cpu.
for example

package main

import (
	"crypto/rand"
	"encoding/binary"
	"fmt"
	"sync"
)

var mcsLock sync.Mutex
var mcsList map[string]bool

func main() {
	mcsList = make(map[string]bool)
	for i:=0;i<1000;i++{
		uniqMcs(10)
	}
}

func uniqMcs(catRange uint32) string {
	var (
		n      uint32
		c1, c2 uint32
		mcs    string
	)
	for {
		binary.Read(rand.Reader, binary.LittleEndian, &n)
		c1 = n % catRange
		binary.Read(rand.Reader, binary.LittleEndian, &n)
		c2 = n % catRange
		if c1 == c2 {
			continue
		} else {
			if c1 > c2 {
				t := c1
				c1 = c2
				c2 = t
			}
		}
		mcs = fmt.Sprintf("s0:c%d,c%d", c1, c2)
		if err := mcsAdd(mcs); err != nil {
			// if len(mcsList) == int(catRange*(catRange-1)/2) {
			// 	time.Sleep(time.Second)
			// }
			fmt.Println(len(mcsList))
			continue
		}
		break
	}
	return mcs
}

func mcsAdd(mcs string) error {
	mcsLock.Lock()
	defer mcsLock.Unlock()
	fmt.Println(len(mcsList))
	if mcsList[mcs] {
		return fmt.Errorf("MCS Label already exists")
	}
	mcsList[mcs] = true
	return nil
}

[ningmingxiao]

we just need return an error see #246