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.*}

The issue identifies footer reading and Hadoop configuration parallelism as the entry points. Trace where the two ExecutorService instances are created and compare their sizing with the number of Callable footer tasks. Done means avoiding excess threads when fewer tasks exist than the configured parallelism.

Written by the indexing model from the issue text.