[SPARK-4835] [WIP] Disable validateOutputSpecs for Spark Streaming jobs

This patch disables output spec. validation for jobs launched through Spark Streaming, since this interferes with checkpoint recovery.


Hadoop OutputFormats have a `checkOutputSpecs` method which performs certain checks prior to writing output, such as checking whether the output directory already exists.  SPARK-1100 added checks for FileOutputFormat, SPARK-1677 (#947) added a SparkConf configuration to disable these checks, and SPARK-2309 (#1088) extended these checks to run for all OutputFormats, not just FileOutputFormat.

In Spark Streaming, we might have to re-process a batch during checkpoint recovery, so `save` actions may be called multiple times.  In addition to `DStream`'s own save actions, users might use `transform` or `foreachRDD` and call the `RDD` and `PairRDD` save actions.  When output spec. validation is enabled, the second calls to these actions will fail due to existing output.

This patch automatically disables output spec. validation for jobs submitted by the Spark Streaming scheduler and introduces a `spark.streaming.hadoop.validateOutputSpecs` setting to re-enable the old behavior.  This is done by using Scala's `DynamicVariable` to propagate the bypass setting without having to mutate SparkConf or introduce a global variable.
Better to say "jobs generated through Spark Streaming's StreamingContext" rather than "scheduler" and stuff.