Throw exception in HadoopRDD.compute()

[kayousterhout]

We should no longer be using HadoopRDD, because it doesn't use monotasks. To make sure we don't inadvertently use this in experiments, we should throw an exception in HadoopRDD.compute() (and disable any associated tests).

[ccanel]

Should I do this in this, or a new commit?

[kayousterhout]

new commit!

[ccanel]

Should I also remove the ability to save files using the old Hadoop API?

[kayousterhout]

Yeah that sounds great