[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.*}

先找出 ValueVector 以及所有持有 FieldReader 的實作，接著查看相關的 heap footprint issue #412 以了解背景。只有在需要時才建立 reader，且所有受影響的 ValueVector 實作仍然正確，工作才算完成。

由索引模型根據 Issue 內容生成。