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`, but abandoned immediately. Instead of fetching them again via calling `getFileStatus` on each part-file sequentially, we can just pass fetched objects down.

**NOTE** Changes made in this PR is not the most clean solution. We can actually add another overloaded version of `getSplits` that receives a `Map<FileStatus, Footer>` to pass down those `FileStatus` objects. Since `ParquetInputFormat` may face a considerably large change introduced by #2, I'd like to minimize my change for now, and resort to a cleaner solution after #2 is merged.
FYI #2 was merged