Use LRU caching for footers in ParquetInputFormat.

Reopening https://github.com/Parquet/parquet-mr/pull/403 against the new Apache repository.
actually, I'm not sure this isCurrent() method is a good idea.
This is going to produce a lot of requests to the namenode to get the current timestamp of files.
a lot more than is actually required to read the _metadata files.
Parquet files are immutable so I would just assume that they don't change for the lifetime of the job.
That would have unpredictable consequences anyway if they did.