[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.*}

首先定位 Java Avro 转换器及其迭代器，然后阅读每次调用中如何创建并返回 VectorSchemaRoot。当转换器更新并复用同一个 VectorSchemaRoot，使其能够与 filewriter 等组件配合工作时，此更改就完成了；如果有，请运行转换器现有的测试。

由索引模型根据 Issue 内容生成。