Extract the storage operation to an interface from scheduler config

[okJiang]

Enhancement Task

Each scheduler config exist the totally same storage operation like

func (conf *balanceLeaderSchedulerConfig) persistLocked() error {
	data, err := EncodeConfig(conf)
	if err != nil {
		return err
	}
	return conf.storage.SaveSchedulerConfig(BalanceLeaderName, data)
}

and

	cfgData, err := l.conf.storage.LoadSchedulerConfig(l.GetName())
	if err != nil {
		return err
	}
	if len(cfgData) == 0 {
		return nil
	}
	newCfg := &balanceLeaderSchedulerConfig{}
	if err = DecodeConfig([]byte(cfgData), newCfg); err != nil {
		return err
	}

It is better to unify them to reduce redundant code