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.*}

首先定位 Statistics 类以及现有的原始类型专用方法，例如 writeInt、writeLong 和 writeDouble。将当前的 InstStatistics、LongStatistics 和 DoubleStatistics 设计与周围的专用模式进行比较。完成的标准是 Statistics 实现遵循一种一致的模式，并且将提议的 ParquetTypeVisitor 接口作为设计的一部分解决。

由索引模型根据 Issue 内容生成。