[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.*}

Comece localizando ValueVector e todas as implementações que contêm um FieldReader; em seguida, consulte a issue relacionada sobre o uso de memória no heap, #412, para obter contexto. O trabalho estará concluído quando o reader for criado somente quando necessário e todas as implementações afetadas de ValueVector continuarem corretas.

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