Parquet pages for different columns cannot be read in parallel

All ColumnChunkPageReader instances use the same decompressor. 

```java

BytesInputDecompressor decompressor = options.getCodecFactory().getDecompressor(descriptor.metadata.getCodec());
return new ColumnChunkPageReader(decompressor, pagesInChunk, dictionaryPage);
```
The CodecFactory caches the decompressors for every codec type returning the same instance on every getCompressor(codecName) call. See the caching happening here:

```java

@Override
public BytesDecompressor getDecompressor(CompressionCodecName codecName) {
BytesDecompressor decomp = decompressors.get(codecName);
if (decomp == null){
decomp = createDecompressor(codecName); decompressors.put(codecName, decomp);
}
return decomp;
}
 
```
 

If multiple threads try to read the pages belonging to different columns, they run into thread

safety issues. This issue prevents increasing the throughput at which applications can read parquet data by parallelizing page reads. 

**Reporter**: [Samarth Jain](https://issues.apache.org/jira/secure/ViewProfile.jspa?name=samarthjain) / @samarthjain
**Assignee**: [Samarth Jain](https://issues.apache.org/jira/secure/ViewProfile.jspa?name=samarthjain) / @samarthjain
#### PRs and other links:
- [GitHub Pull Request #668](https://github.com/apache/parquet-mr/pull/668)
- [GitHub Pull Request #670](https://github.com/apache/parquet-mr/pull/670)

_{**Note**: *This issue was originally created as [PARQUET-1641](https://issues.apache.org/jira/browse/PARQUET-1641). Please see the [migration documentation](https://issues.apache.org/jira/browse/PARQUET-2502) for further details.*}

Inizia con ParquetFileReader.java intorno alla riga 1286 e CodecFactory.java intorno alla riga 197 per tracciare il modo in cui i decompressori vengono condivisi dalle istanze di ColumnChunkPageReader. Esamina le pull request #668 e #670 prima di iniziare, quindi verifica che le letture delle pagine per colonne diverse possano essere eseguite in parallelo senza il problema di thread safety segnalato.

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