func (c *Cluster) AddMember(id uint64, url string) {
	c.mutex.Lock()
	defer c.mutex.Unlock()
	c.members[id] = url
}
AddMember add a new member to Cluster
package casbinraft
import (
	"sync"
	"github.com/coreos/etcd/raft/raftpb"
)
type Cluster struct {
	members map[uint64]string
	mutex   sync.RWMutex
}
func NewCluster(peers map[uint64]string) *Cluster {
	return &Cluster{
		members: peers,
	}
}
func (c *Cluster) GetURL(id uint64) string {
	c.mutex.RLock()
	defer c.mutex.RUnlock()
	return c.members[id]
}