[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.*}

首先定位 ValueVector 以及所有持有 FieldReader 的实现，然后查看相关的堆内存占用 issue #412 以了解背景。只有在需要时才创建 reader，并且所有受影响的 ValueVector 实现仍然正确，这项工作才算完成。

由索引模型根据 Issue 内容生成。