InternalParquetRecordWriter doesn't immediately limit current row group to threshold

The MemoryManager adjust the row group size threshold of writers when the allocated memory pool fills up.
**Problem**: However InternalParquetRecordWriter only re-adjusts the row group size on the next flush meaning they still use the old size.
This opens up a possibility of getting an OOM error if all writers are started at relatively the same time and progress in tandem(I saw this when investigating failing jobs while writing to disk in Spark)

**Reporter**: [Brian Mwambazi](https://issues.apache.org/jira/secure/ViewProfile.jspa?name=brimzi) / @brimzi
#### PRs and other links:
- [GitHub Pull Request #748](https://github.com/apache/parquet-mr/pull/748)

_{**Note**: *This issue was originally created as [PARQUET-1767](https://issues.apache.org/jira/browse/PARQUET-1767). Please see the [migration documentation](https://issues.apache.org/jira/browse/PARQUET-2502) for further details.*}

Inizia esaminando GitHub Pull Request #748 e gli entry point InternalParquetRecordWriter e MemoryManager menzionati nell’issue. Conferma come viene aggiornato il threshold del row group quando il memory pool si riempie e verifica che i writer smettano immediatamente di usare il threshold precedente, così che i writer paralleli non corrano il rischio di causare un OOM.

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