[SPARK-2108] Mark SparkContext methods that return block information as developer API's

Author: Prashant Sharma <prashant.s@imaginea.com>

Closes #1047 from ScrapCodes/SPARK-2108/mark-as-dev-api and squashes the following commits:

073ee34 [Prashant Sharma] [SPARK-2108] Mark SparkContext methods that return block information as developer API's
(cherry picked from commit e508f599f88baaa31a3498fb0bdbafdbc303119e)

Signed-off-by: Patrick Wendell <pwendell@gmail.com>
SPARK-2113: awaitTermination() after stop() will hang in Spark Stremaing

Author: Lars Albertsson <lalle@spotify.com>

Closes #1001 from lallea/contextwaiter_stopped and squashes the following commits:

93cd314 [Lars Albertsson] Mend StreamingContext stop() followed by awaitTermination().
(cherry picked from commit 4d5c12aa1c54c49377a4bafe3bcc4993d5e1a552)

Signed-off-by: Patrick Wendell <pwendell@gmail.com>