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.*}

首先跟踪 Parquet 记录读取路径，并将其与 Avro DatumReader 的对象复用进行比较，然后确定作业配置如何控制该行为。完成标准是：读取记录时，reader 可以选择性地复用对象，并且该设置和预期行为已定义并由适当的测试覆盖。

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