[Java] remove FieldReader from ValueVector

Every implementation of ValueVector has an instance of .FieldReader, which has an overhead of 28 bytes on the heap. This can be avoided by instantiating the object only when required.

**Reporter**: [Pindikura Ravindra](https://issues.apache.org/jira/browse/ARROW-5484) / @pravindra
#### Related issues:
- [[Java] reduce heap footprint of ValueVectors](https://github.com/apache/arrow-java/issues/412) (is a child of)

_{**Note**: *This issue was originally created as [ARROW-5484](https://issues.apache.org/jira/browse/ARROW-5484). Please see the [migration documentation](https://github.com/apache/arrow/issues/14542) for further details.*}

Inizia individuando ValueVector e ogni implementazione che contiene un FieldReader, quindi esamina la issue correlata sull’heap footprint, #412, per avere il contesto. Il lavoro è completato quando il reader viene creato solo quando necessario e tutte le implementazioni di ValueVector interessate rimangono corrette.

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