[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，并将其对 VectorSchemaRoot 的处理方式与 issue 中提到的 filewriter 组件进行比较。确认 iterator 当前如何创建 roots，并找出相关测试。当 converter 复用一个 VectorSchemaRoot，并能与要求单个 root 的组件配合工作时，即表示完成。

由索引模型根据 Issue 内容生成。