[SPARK-3250] Implement Gap Sampling optimization for random sampling

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

need a reference in comments, e.g., http://en.wikipedia.org/wiki/Poisson_distribution#Generating_Poisson-distributed_random_variables