Allow concurrent writes to different partitions with dynamic partition overwrite

It is desirable to run concurrent jobs that write to different partitions within same baseDir using partitionBy and dynamic partitionOverwriteMode.

See for example here:
https://stackoverflow.com/questions/38964736/multiple-spark-jobs-appending-parquet-data-to-same-base-path-with-partitioning

Or the discussion here:
https://github.com/delta-io/delta/issues/9

This doesnt seem that difficult. I suspect only changes needed are in org.apache.spark.internal.io.HadoopMapReduceCommitProtocol, which already has a flag for dynamicPartitionOverwrite. I got a quick test to work by disabling all committer activity (committer.setupJob, committer.commitJob, etc.) when dynamicPartitionOverwrite is true.