Also lays the groundwork for supporting it inside the external shuffle service.

Author: Aaron Davidson <aaron@databricks.com>

Closes #3087 from aarondav/sasl and squashes the following commits:

3481718 [Aaron Davidson] Delete rogue println
44f8410 [Aaron Davidson] Delete documentation - muahaha!
eb9f065 [Aaron Davidson] Improve documentation and add end-to-end test at Spark-level
a6b95f1 [Aaron Davidson] Address comments
785bbde [Aaron Davidson] Cleanup
79973cb [Aaron Davidson] Remove unused file
151b3c5 [Aaron Davidson] Add docs, timeout config, better failure handling
f6177d7 [Aaron Davidson] Cleanup SASL state upon connection termination
7b42adb [Aaron Davidson] Add unit tests
8191bcb [Aaron Davidson] [SPARK-2938] Support SASL authentication in NettyBlockTransferService