[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.*}

まず ValueVector と FieldReader を保持するすべての実装を見つけ、関連するヒープフットプリントの issue #412 を確認して背景を把握します。reader が必要な場合にのみ作成され、影響を受けるすべての ValueVector 実装が引き続き正しく動作すれば、作業は完了です。

索引モデルが issue の本文から書いたものです。