Improve the creation of ExecutorService when reading footers

Doing some benchmarks loading a large set of parquet files (3000+) from the local FS, we observed some inefficiencies in the number of created threads when reading footers.

By reading, the read the configuration parallelism in Hadoop configuration (defaulted to 5) and allocate 2 ExecuteService with each 5 threads to read footers. This is especially inefficient if there is less Callable to handle than the configured parallelism.

**Reporter**: [Benoit Lacelle](https://issues.apache.org/jira/secure/ViewProfile.jspa?name=blasd)

_{**Note**: *This issue was originally created as [PARQUET-1055](https://issues.apache.org/jira/browse/PARQUET-1055). Please see the [migration documentation](https://issues.apache.org/jira/browse/PARQUET-2502) for further details.*}

该 issue 将 footer 读取和 Hadoop 配置并行度确定为切入点。追踪两个 ExecutorService 实例的创建位置，并将其大小与 Callable footer 任务的数量进行比较。当任务数少于配置的并行度时能够避免创建多余线程，即表示完成。

由索引模型根据 Issue 内容生成。