[SPARK-6752] [STREAMING] [REVISED] Allow StreamingContext to be recreated from...

[SPARK-6752] [STREAMING] [REVISED] Allow StreamingContext to be recreated from checkpoint and existing SparkContext

This is a revision of the earlier version (see #5773) that passed the active SparkContext explicitly through a new set of Java and Scala API. The drawbacks are.

* Hard to implement in python.
* New API introduced. This is even more confusing since we are introducing getActiveOrCreate in SPARK-7553

Furthermore, there is now a direct way get an existing active SparkContext or create a new on - SparkContext.getOrCreate(conf). Its better to use this to get the SparkContext rather than have a new API to explicitly pass the context.

So in this PR I have
* Removed the new versions of StreamingContext.getOrCreate() which took SparkContext
* Added the ability to pick up existing SparkContext when the StreamingContext tries to create a SparkContext.

Author: Tathagata Das <tathagata.das1565@gmail.com>

Closes #6096 from tdas/SPARK-6752 and squashes the following commits:

53f4b2d [Tathagata Das] Merge remote-tracking branch 'apache-github/master' into SPARK-6752
f024b77 [Tathagata Das] Removed extra API and used SparkContext.getOrCreate