[SPARK-6717][ML] Clear shuffle files after checkpointing in ALS

## What changes were proposed in this pull request?

When ALS is run with a checkpoint interval, during the checkpoint materialize the current state and cleanup the previous shuffles (non-blocking).

## How was this patch tested?

Existing ALS unit tests, new ALS checkpoint cleanup unit tests added & shuffle files checked after ALS w/checkpointing run.

Author: Holden Karau <holden@us.ibm.com>
Author: Holden Karau <holden@pigscanfly.ca>

Closes #11919 from holdenk/SPARK-6717-clear-shuffle-files-after-checkpointing-in-ALS.