[Java] JDBC to Arrow converter iterator 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 JDBC 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-9614) / @emkornfield

_{**Note**: *This issue was originally created as [ARROW-9614](https://issues.apache.org/jira/browse/ARROW-9614). Please see the [migration documentation](https://github.com/apache/arrow/issues/14542) for further details.*}

首先定位 Java JDBC converter 的 iterator，並將其對 VectorSchemaRoot 的處理方式與 issue 中提到的 filewriter 元件進行比較。確認 iterator 目前如何建立 roots，並找出相關測試。當 converter 重複使用一個 VectorSchemaRoot，並能與要求單一 root 的元件搭配運作時，即表示完成。

由索引模型根據 Issue 內容生成。