[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.*}

Comienza localizando el iterador del Java JDBC converter y compara su manejo de VectorSchemaRoot con los componentes filewriter mencionados en el issue. Confirma cómo crea actualmente los roots el iterador e identifica las pruebas relacionadas. La tarea está terminada cuando el converter reutiliza un único VectorSchemaRoot y funciona con componentes que esperan un solo root.

Escrito por el modelo de indexación a partir del texto del issue.