Perf: Optimize the loading of StatisticsMetaLoader

[KKould]

Feature Request

As mentioned in issue: #162, StatisticsMetaLoader::load takes a considerable amount of time.
Currently, StatisticsMetaLoader loads table statistical information at a granularity. This may happen when a table has only a few indexes that are frequently used, but indexes that are not commonly used are still loaded. We can reduce the granularity and increase the loading speed.

Tips: StatisticsMeta is based on Index as the granularity

pub fn load(&self, table_name: TableName) -> Result<&Vec<StatisticsMeta>, DatabaseError> {
        let option = self.cache.get(&table_name);

        if let Some(statistics_metas) = option {
            Ok(statistics_metas)
        } else {
            let paths = self.tx.statistics_meta_paths(&table_name)?;
            let mut statistics_metas = Vec::with_capacity(paths.len());

            for path in paths {
                statistics_metas.push(StatisticsMeta::from_file(path)?);
            }

            Ok(self
                .cache
                .get_or_insert(table_name, |_| Ok(statistics_metas))?)
        }
    }