stream through files when writing the _metadata file to reduce memory usage

Currently we load all the footers of the part files in memory in the committer to write the _metadata file.
We could open the _metadata file first and then add the metadata for each part file one by one. That would reduce the required memory.

**Reporter**: [Julien Le Dem](https://issues.apache.org/jira/secure/ViewProfile.jspa?name=julienledem) / @julienledem

_{**Note**: *This issue was originally created as [PARQUET-91](https://issues.apache.org/jira/browse/PARQUET-91). Please see the [migration documentation](https://issues.apache.org/jira/browse/PARQUET-2502) for further details.*}