Simple PageRank algorithm implementation in Java for SPARK-760
Updated code with reviewer's suggestions
[HOTFIX] Extend thread safety for SparkEnv.get()

A ThreadLocal SparkEnv.env is facing various situations leading to
NullPointerExceptions, where SparkEnv.env set in one thread is not
gettable in another thread, but often assumed to be available.

See, e.g., https://groups.google.com/forum/#!topic/spark-developers/GLx8yunSj0A

This hotfixes SparkEnv.env to return either (a) the ThreadLocal
value if non-null, or (b) the previously set value in any thread.

This approach preserves SparkEnv.set() thread safety needed by
RDD.compute() and possibly other places. A refactoring that
parameterizes SparkEnv should be addressed subsequently.

On branch adatao-global-SparkEnv
Changes to be committed:

	modified:   core/src/main/scala/spark/SparkEnv.scala
[HOTFIX] Mark lastSetSparkEnv @volatile in case it gets HotSpot-cached

On branch adatao-global-SparkEnv
Changes to be committed:

	modified:   core/src/main/scala/spark/SparkEnv.scala
changes as reviewer requested
Got rid of unnecessary map function
Merge pull request #770 from stayhf/SPARK-760-Java

Simple PageRank algorithm implementation in Java for SPARK-760
Merge pull request #779 from adatao/adatao-global-SparkEnv

[HOTFIX] Extend thread safety for SparkEnv.get()
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