[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.*}

Inizia individuando il convertitore Java Avro e il suo iteratore, quindi leggi come VectorSchemaRoot viene creato e restituito a ogni chiamata. La modifica è completata quando il convertitore aggiorna e riutilizza un unico VectorSchemaRoot, consentendogli di funzionare con componenti come filewriter; esegui i test esistenti del convertitore, se disponibili.

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