SPARK-3699:  SQL and Hive console tasks now clean up appropriately

The sbt tasks sql/console and hive/console will now `stop()`
the `SparkContext` upon exit.  Previously, they left an ugly stack
trace when quitting.

Author: William Benton <willb@redhat.com>

Closes #2547 from willb/consoleCleanup and squashes the following commits:

d5e431f [William Benton] SQL and Hive console tasks now clean up.