[SPARK-7478] [SQL] Added SQLContext.getOrCreate

Having a SQLContext singleton would make it easier for applications to use a lazily instantiated single shared instance of SQLContext when needed. It would avoid problems like

1. In REPL/notebook environment, rerunning the line {{val sqlContext = new SQLContext}} multiple times created different contexts while overriding the reference to previous context, leading to issues like registered temp tables going missing.

2. In Streaming, creating SQLContext directly leads to serialization/deserialization issues when attempting to recover from DStream checkpoints. See [SPARK-6770]. Also to get around this problem I had to suggest creating a singleton instance - https://github.com/apache/spark/blob/master/examples/src/main/scala/org/apache/spark/examples/streaming/SqlNetworkWordCount.scala

This can be solved by {{SQLContext.getOrCreate}} which get or creates a new singleton instance of SQLContext using either a given SparkContext or a given SparkConf.

rxin marmbrus

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

Closes #6006 from tdas/SPARK-7478 and squashes the following commits:

25f4da9 [Tathagata Das] Addressed comments.
79fe069 [Tathagata Das] Added comments.
c66ca76 [Tathagata Das] Merge remote-tracking branch 'apache-github/master' into SPARK-7478
48adb14 [Tathagata Das] Removed HiveContext.getOrCreate
bf8cf50 [Tathagata Das] Fix more bug
dec5594 [Tathagata Das] Fixed bug
b4e9721 [Tathagata Das] Remove unnecessary import
4ef513b [Tathagata Das] Merge remote-tracking branch 'apache-github/master' into SPARK-7478
d3ea8e4 [Tathagata Das] Added HiveContext
83bc950 [Tathagata Das] Updated tests
f82ae81 [Tathagata Das] Fixed test
bc72868 [Tathagata Das] Added SQLContext.getOrCreate

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