More efficient sampling, based on Gap Sampling optimization:
http://erikerlandson.github.io/blog/2014/09/11/faster-random-samples-with-gap-sampling/

Author: Erik Erlandson <eerlands@redhat.com>

Closes #2455 from erikerlandson/spark-3250-pr and squashes the following commits:

72496bc [Erik Erlandson] [SPARK-3250] Implement Gap Sampling optimization for random sampling