Scope all RDD methods

This commit provides a mechanism to set and unset the call scope
around each RDD operation defined in RDD.scala. This is useful
for tagging an RDD with the scope in which it is created. This
will be extended to similar methods in SparkContext.scala and
other relevant files in a future commit.

Author: Andrew Or

core/src/main/scala/org/apache/spark/SparkContext.scala

@@ -1983,10 +1983,10 @@ object SparkContext extends Logging {
   }
 
   private[spark] val SPARK_JOB_DESCRIPTION = "spark.job.description"
-
   private[spark] val SPARK_JOB_GROUP_ID = "spark.jobGroup.id"
-
   private[spark] val SPARK_JOB_INTERRUPT_ON_CANCEL = "spark.job.interruptOnCancel"
+  private[spark] val RDD_SCOPE_KEY = "spark.rdd.scope"
+  private[spark] val RDD_SCOPE_NO_OVERRIDE_KEY = "spark.rdd.scope.noOverride"
 
   /**
    * Executor id for the driver.  In earlier versions of Spark, this was `<driver>`, but this was