。心跳信息由master在主机binlog日志文件在设定的间隔时间内没有收到新的事件时发出，以便slave知道master是否正常
slave_net_timeout 是设置在多久没收到数据后认为网络超时，之后 Slave 的 IO 线程会重新连接 Master
Slave_heartbeat_period
Shows the replication heartbeat interval (in seconds) on a replication slave.
Slave_last_heartbeat
Shows when the most recent heartbeat signal was received by a replication slave, as a TIMESTAMP value.
Slave_received_heartbeats
This counter increments with each replication heartbeat received by a replication slave since the last time that the slave was restarted or reset, or a CHANGE MASTER TO statement was issued.
