CDK-492: Add DurableParquetAppender.

This appender wraps an AvroAppender and a ParquetAppender to provide
durability by writing to Avro in tandem with writing parquet files
directly. After the file is closed, the avro copy is removed in a new
cleanup phase in the appender. Any failure requires recovery using the
Avro files.
Since they should not be touched in most cases, I like having them in the SPI. We should document them in an advanced configuration section at some point. I filed [CDK-523](https://issues.cloudera.org/browse/CDK-523) to track.