Reduce memory pressure when reading footers

I encountered OOMs reading metadata for a dataset with 500+ columns, many of them quite sparse.

We can reduce memory utilization significantly.

**Reporter**: [Dmitriy V. Ryaboy](https://issues.apache.org/jira/secure/ViewProfile.jspa?name=dvryaboy) / @dvryaboy

_{**Note**: *This issue was originally created as [PARQUET-11](https://issues.apache.org/jira/browse/PARQUET-11). Please see the [migration documentation](https://issues.apache.org/jira/browse/PARQUET-2502) for further details.*}

首先重现读取具有 500 多个稀疏列的数据集元数据时的内存不足情况，然后跟踪 footer 读取路径。该 issue 没有指出任何文件或测试，因此请先确定相关的元数据和 footer 代码，再决定如何修改。完成的标准是在不改变元数据读取结果的情况下，显著降低内存使用量。

由索引模型根据 Issue 内容生成。