redis 4.0 cluster方式

每隔一段时间去检查各个节点的状态，主要的函数如下：

cluster node信息的存放位置：server.cluster->nodes

server.c ->serverCron->

This is executed 10 times every second

cluster.c->clusterCron

遍历nodes的后通过网络的连接查看节点是否存活

fd = anetTcpNonBlockBindConnect(server.neterr, node->ip,node->cport, NET_FIRST_BIND_ADDR);

放入nio监听事件循环

aeCreateFileEvent(server.el,link->fd,AE_READABLE,clusterReadHandler,link);

发送ping命令

clusterSendPing(link, node->flags & CLUSTER_NODE_MEET ?CLUSTERMSG_TYPE_MEET : CLUSTERMSG_TYPE_PING);