[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.*}

Inizia individuando l’iteratore del Java JDBC converter e confronta la sua gestione di VectorSchemaRoot con quella dei componenti filewriter menzionati nell’issue. Conferma come l’iteratore crea attualmente i roots e identifica i test correlati. Il lavoro è completato quando il converter riutilizza un unico VectorSchemaRoot e funziona con componenti che si aspettano un solo root.

Scritto dal modello di indicizzazione a partire dal testo della issue.