Reservoir sampling implementation.

This is going to be used in https://issues.apache.org/jira/browse/SPARK-2568

Author: Reynold Xin <rxin@apache.org>

Closes #1478 from rxin/reservoirSample and squashes the following commits:

17bcbf3 [Reynold Xin] Added seed.
badf20d [Reynold Xin] Renamed the method.
6940010 [Reynold Xin] Reservoir sampling implementation.