Add option to reuse (Avro) objects when reading

Instead of creating an object for each record, Avro DatumReaders support passing in an object to be reused. It would be nice to expose this in Parquet.

This could be turned on with a setting in the job configuration.

**Reporter**: [Sandy Ryza](https://issues.apache.org/jira/secure/ViewProfile.jspa?name=sandyr) / @sryza

_{**Note**: *This issue was originally created as [PARQUET-5](https://issues.apache.org/jira/browse/PARQUET-5). Please see the [migration documentation](https://issues.apache.org/jira/browse/PARQUET-2502) for further details.*}

Start by tracing the Parquet record-reading path and comparing it with Avro DatumReader object reuse, then identify how job configuration could control the behavior. Done means readers can optionally reuse objects when reading records, with the setting and expected behavior defined and covered by appropriate tests.

Written by the indexing model from the issue text.