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PARQUET-1780: [C++] Set ColumnMetadata.encoding_stats field

This is to solve the issue PARQUET-1780:
ColumnMetadata.encoding_stats field is empty in parquet-cpp implementation.
This leads to metadata mismatches between 2 parquet files generated by cpp and scala(parquet-mr).

encoding_stat is a vector of **PageEncodingStats**.
PageEncodingStats has three attributes:

- page_type: (data or dict)
- encoding: encoding of the page
- count:number of pages of this type with this encoding

From above first to can be extracted from available information. But for count I have to create a add some attributes to exisiting classes.
Modifications:
For the class **SerializedPageWriter**, added following two attributes.
  int32_t num_dict_pages_;
  std::pair<int32_t, int32_t> num_data_pages_; (first: number of un-encoded pages,
                                                                            second:number of encoded pages )

Closes #6370 from omega-gamage/PARQUET-1780 and squashes the following commits:

086af4e8d <Wes McKinney> Code review comments
a9c684b25 <Omega Gamage> Match the implementation with impala implementation
eae56fa4b <Wes McKinney> Simplify PageEncodingStats
54ac1eb15 <Omega Gamage> commit 9eecaaf5fe895f85d5352ec7420267701d6d6e8f Author: Omega Gamage <omega@bigstream.co> Date:   Tue Feb 18 14:23:08 2020 +0530

Lead-authored-by: Omega Gamage <omega@bigstream.co>
Co-authored-by: Wes McKinney <wesm+git@apache.org>
Signed-off-by: Wes McKinney <wesm+git@apache.org>