Refactor the Statistics classes to match the specialized pattern used throughout parquet

Because Parquet tries very hard to avoid autoboxing, most of the core classes are specialized for each primitive by having a method for each type, eg:

```Java
void writeInt(int x);
void writeLong(long x);
void writeDouble(double x);
```

and so on.

However, the statistics classes take the other approach of having an InstStatistics class, a LongStatistics class, a DoubleStatistics class and so on. I think it's worth going for consistency and picking a pattern and sticking to it. Seems like the first pattern I mentioned is currently the more common one.

We may want to take this one step further and define an interface that these all conform to, eg:
```Java
public interface ParquetTypeVisitor {
void visitInt(int x);
void visitLong(long x);
void visitDouble(double x);
}
```

**Reporter**: [Alex Levenson](https://issues.apache.org/jira/secure/ViewProfile.jspa?name=alexlevenson) / @isnotinvain

_{**Note**: *This issue was originally created as [PARQUET-32](https://issues.apache.org/jira/browse/PARQUET-32). Please see the [migration documentation](https://issues.apache.org/jira/browse/PARQUET-2502) for further details.*}

まず、Statistics クラスと、writeInt、writeLong、writeDouble などの既存のプリミティブ型特化メソッドを見つけます。現在の InstStatistics、LongStatistics、DoubleStatistics の設計を、周辺にある特化パターンと比較します。Statistics の実装が一貫したパターンに従い、提案された ParquetTypeVisitor インターフェースが設計の一部として解決されれば完了です。

索引モデルが issue の本文から書いたものです。