[Java] Avro to Arrow converter should reuse the same VectorSchemaRoot

When originally reviewing the code I suggested a new VectorSchemaRoot on each call to the iterator.  After further discussions on the mailing list, it seems that this is an anit-pattern for working with VectorSchemaRoot, we should update the code to update a single VectorSchemaRoot.

After this change it should be easier to use Avro converter with other components of the library (i.e. filewriter) which also make use of a single VectorSchemaRoot.

**Reporter**: [Micah Kornfield](https://issues.apache.org/jira/browse/ARROW-9613) / @emkornfield

_{**Note**: *This issue was originally created as [ARROW-9613](https://issues.apache.org/jira/browse/ARROW-9613). Please see the [migration documentation](https://github.com/apache/arrow/issues/14542) for further details.*}

先找出 Java Avro 轉換器及其 iterator，接著閱讀每次呼叫如何建立並回傳 VectorSchemaRoot。當轉換器更新並重複使用同一個 VectorSchemaRoot，使其能夠與 filewriter 等元件搭配運作時，這項變更就完成了；如果有，請執行轉換器現有的測試。

由索引模型根據 Issue 內容生成。