This patch builds directly on #7820, which is largely written by tnachen. The only addition is one commit for cleaning up the code. There should be no functional differences between this and #7820.

Author: Timothy Chen <tnachen@gmail.com>
Author: Andrew Or <andrew@databricks.com>

Closes #7881 from andrewor14/tim-cleanup-mesos-shuffle and squashes the following commits:

8894f7d [Andrew Or] Clean up code
2a5fa10 [Andrew Or] Merge branch 'mesos_shuffle_clean' of github.com:tnachen/spark into tim-cleanup-mesos-shuffle
fadff89 [Timothy Chen] Address comments.
e4d0f1d [Timothy Chen] Clean up external shuffle data on driver exit with Mesos.