[SPARK-6752][Streaming] Allow StreamingContext to be recreated from checkpoint...

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

Currently if you want to create a StreamingContext from checkpoint information, the system will create a new SparkContext. This prevent StreamingContext to be recreated from checkpoints in managed environments where SparkContext is precreated.

The solution in this PR: Introduce the following methods on StreamingContext
1. `new StreamingContext(checkpointDirectory, sparkContext)`
   Recreate StreamingContext from checkpoint using the provided SparkContext
2. `StreamingContext.getOrCreate(checkpointDirectory, sparkContext, createFunction: SparkContext => StreamingContext)`
   If checkpoint file exists, then recreate StreamingContext using the provided SparkContext (that is, call 1.), else create StreamingContext using the provided createFunction

TODO: the corresponding Java and Python API has to be added as well.

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

Closes #5428 from tdas/SPARK-6752 and squashes the following commits:

94db63c [Tathagata Das] Fix long line.
524f519 [Tathagata Das] Many changes based on PR comments.
eabd092 [Tathagata Das] Added Function0, Java API and unit tests for StreamingContext.getOrCreate
36a7823 [Tathagata Das] Minor changes.
204814e [Tathagata Das] Added StreamingContext.getOrCreate with existing SparkContext