[SPARK-6437][SQL] Use completion iterator to close external sorter

Otherwise we will leak files when spilling occurs.

Author: Michael Armbrust <michael@databricks.com>

Closes #5161 from marmbrus/cleanupAfterSort and squashes the following commits:

cb13d3c [Michael Armbrust] hint to inferencer
cdebdf5 [Michael Armbrust] Use completion iterator to close external sorter