Author: Reynold Xin <rxin@databricks.com>

Closes #5709 from rxin/inc-id and squashes the following commits:

7853611 [Reynold Xin] private sql.
a9fda0d [Reynold Xin] Missed a few numbers.
343d896 [Reynold Xin] Self review feedback.
a7136cb [Reynold Xin] [SPARK-7135][SQL] DataFrame expression for monotonically increasing IDs.