[SPARK-3015] Block on cleaning tasks to prevent Akka timeouts

More detail on the issue is described in [SPARK-3015](https://issues.apache.org/jira/browse/SPARK-3015), but the TLDR is if we send too many blocking Akka messages that are dependent on each other in quick successions, then we end up causing a few of these messages to time out and ultimately kill the executors. As of #1498, we broadcast each RDD whether or not it is persisted. This means if we create many RDDs (each of which becomes a broadcast) and the driver performs a GC that cleans up all of these broadcast blocks, then we end up sending many `RemoveBroadcast` messages in parallel and trigger the chain of blocking messages at high frequencies.

We do not know of the Akka-level root cause yet, so this is intended to be a temporary solution until we identify the real issue. I have done some preliminary testing of enabling blocking and observed that the queue length remains quite low (< 1000) even under very intensive workloads.

In the long run, we should do something more sophisticated to allow a limited degree of parallelism through batching clean up tasks or processing them in a sliding window. In the longer run, we should clean up the whole `BlockManager*` message passing interface to avoid unnecessarily awaiting on futures created from Akka asks.

tdas pwendell mengxr

Author: Andrew Or <andrewor14@gmail.com>

Closes #1931 from andrewor14/reference-blocking and squashes the following commits:

d0f7195 [Andrew Or] Merge branch 'master' of github.com:apache/spark into reference-blocking
ce9daf5 [Andrew Or] Remove logic for logging queue length
111192a [Andrew Or] Add missing space in log message (minor)
a183b83 [Andrew Or] Switch order of code blocks (minor)
9fd1fe6 [Andrew Or] Remove outdated log
104b366 [Andrew Or] Use the actual reference queue length
0b7e768 [Andrew Or] Block on cleaning tasks by default + log error on queue full

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

@@ -66,10 +66,15 @@ private[spark] class ContextCleaner(sc: SparkContext) extends Logging {
 
   /**
    * Whether the cleaning thread will block on cleanup tasks.
-   * This is set to true only for tests.
+   *
+   * Due to SPARK-3015, this is set to true by default. This is intended to be only a temporary
+   * workaround for the issue, which is ultimately caused by the way the BlockManager actors
+   * issue inter-dependent blocking Akka messages to each other at high frequencies. This happens,
+   * for instance, when the driver performs a GC and cleans up all broadcast blocks that are no
+   * longer in scope.
    */
   private val blockOnCleanupTasks = sc.conf.getBoolean(
-    "spark.cleaner.referenceTracking.blocking", false)
+    "spark.cleaner.referenceTracking.blocking", true)
 
   @volatile private var stopped = false
 
@@ -174,9 +179,6 @@ private[spark] class ContextCleaner(sc: SparkContext) extends Logging {
   private def blockManagerMaster = sc.env.blockManager.master
   private def broadcastManager = sc.env.broadcastManager
   private def mapOutputTrackerMaster = sc.env.mapOutputTracker.asInstanceOf[MapOutputTrackerMaster]
-
-  // Used for testing. These methods explicitly blocks until cleanup is completed
-  // to ensure that more reliable testing.
 }
 
 private object ContextCleaner {