[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.*}

首先定位 issue 中描述的基於 delegate 的 consumer 和手寫的 consumer 路徑。比較它們的效能，然後評估是否有必要使用 converter 產生的自訂類別；完成的標準是形成一份有文件記錄的比較結果，並作出範圍明確的實作決策。

由索引模型根據 Issue 內容生成。