Failover step

StepBeginExchange	启动切换
StepOldMasterSetReadonly	旧主库停写
StepPrepareExchange	从库停同步(本地回放)
StepMakeupData	补数据(非必须执行流程)
StepElectMaster	选主
StepChangeNewMaster	从库切至新主
StepNewMasterFinish	新主库开写
StepUpdateTopo	更新拓扑
StepFinishExchange	切换完毕