Difference between parquet-mr implementation and parquet-format documentation

Documentation at https://github.com/apache/parquet-format/blob/master/src/thrift/parquet.thrift

```
struct ColumnChunk {
/** File where column data is stored. If not set, assumed to be same file as
* metadata. This path is relative to the current file.
**/
1: optional string file_path

/** Byte offset in file_path to the ColumnMetaData **/
2: required i64 file_offset

...
```

and https://github.com/apache/parquet-format

```
4-byte magic number "PAR1"

...
```

suggests that ColumnChunk data should be followed by ColumnChunkMetaData.

However it looks like parquet-mr doesn't write ColumnMetaData after Columns at all and populates ColumnChunk.file_offset with an offset of the first data page:

from **ParquetMetadataConverter.java:153**:
```Java
for (ColumnChunkMetaData columnMetaData : columns) {
ColumnChunk columnChunk = new ColumnChunk(columnMetaData.getFirstDataPageOffset()); // verify this is the right offset
columnChunk.file_path = block.getPath(); // they are in the same file for now
```

Is it a bug in parquet-mr or in the documentation?

**Reporter**: [Konstantin Shaposhnikov](https://issues.apache.org/jira/secure/ViewProfile.jspa?name=k.shaposhnikov@gmail.com) / @kostya-sh
#### PRs and other links:
- [parquet-format PR #56](https://github.com/apache/parquet-format/pull/56)

_{**Note**: *This issue was originally created as [PARQUET-291](https://issues.apache.org/jira/browse/PARQUET-291). Please see the [migration documentation](https://issues.apache.org/jira/browse/PARQUET-2502) for further details.*}

Inizia da ParquetMetadataConverter.java alla riga 153 e confronta la costruzione di ColumnChunk con la definizione di ColumnChunk in parquet.thrift e la documentazione del layout di parquet-format. Esamina la PR #56 di parquet-format collegata e la documentazione sulla migrazione referenziata nell'issue. Il lavoro è completato quando la discrepanza tra implementazione e documentazione ha una risoluzione chiaramente registrata.

Scritto dal modello di indicizzazione a partire dal testo della issue.