PARQUET-16: Avoid calling getFileStatus() on all part-files

JIRA issue: [PARQUET-16](https://issues.apache.org/jira/browse/PARQUET-16)

When calling `getSplits(JobContext)`, all the `FileStatus` objects are already fetched via `listStatus`. Instead of fetching them via calling `getFileStatus` on each part-file sequentially, we can just pass fetched objects down.