SPARK-5848: tear down the ConsoleProgressBar timer

The timer is a GC root, and failing to terminate it leaks SparkContext
instances.

Author: Matt Whelan <mwhelan@perka.com>

Closes #4635 from MattWhelan/SPARK-5848 and squashes the following commits:

2a1e8a5 [Matt Whelan] SPARK-5848: teardown the ConsoleProgressBar timer

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

@@ -1363,6 +1363,7 @@ class SparkContext(config: SparkConf) extends Logging with ExecutorAllocationCli
         cleaner.foreach(_.stop())
         dagScheduler.stop()
         dagScheduler = null
+        progressBar.foreach(_.stop())
         taskScheduler = null
         // TODO: Cache.stop()?
         env.stop()

core/src/main/scala/org/apache/spark/ui/ConsoleProgressBar.scala

@@ -28,7 +28,6 @@ import org.apache.spark._
  * of them will be combined together, showed in one line.
  */
 private[spark] class ConsoleProgressBar(sc: SparkContext) extends Logging {
-
   // Carrige return
   val CR = '\r'
   // Update period of progress bar, in milliseconds
@@ -121,4 +120,10 @@ private[spark] class ConsoleProgressBar(sc: SparkContext) extends Logging {
     clear()
     lastFinishTime = System.currentTimeMillis()
   }
+
+  /**
+   * Tear down the timer thread.  The timer thread is a GC root, and it retains the entire
+   * SparkContext if it's not terminated.
+   */
+  def stop(): Unit = timer.cancel()
 }