[Java] Avro - Experiment with "compiled" consumer delegates for performance.

All consumers that rely on delegates (e.g. struct, composite, list, union, ...) require  megamorphic lookups which can't be inlined well by JIT.  

We should verify the performance different of a hand-coded consumer vs an existing delegate consumer

i.e. something like:

void consume(Decoder d) {

  ((IntConsumer)delegate).consume(d);

}

compared to the existing implementation.  It is expected we will see a decent amount of performance improvement from this approach.  If we do, we should add an option to converter to generate new custom classes on the fly, that mimic the hand-coded option.

**Reporter**: [Micah Kornfield](https://issues.apache.org/jira/browse/ARROW-6595) / @emkornfield

_{**Note**: *This issue was originally created as [ARROW-6595](https://issues.apache.org/jira/browse/ARROW-6595). Please see the [migration documentation](https://github.com/apache/arrow/issues/14542) for further details.*}

まず、issueで説明されているdelegateベースのconsumerと手作業で記述されたconsumerの経路を特定します。それらのパフォーマンスを比較し、次にconverterで生成されたカスタムクラスが妥当かどうかを評価します。完了の条件は、比較結果が文書化され、実装方針の範囲が定まっていることです。

索引モデルが issue の本文から書いたものです。