[SPARK-17316][CORE] Make CoarseGrainedSchedulerBackend.removeExecutor non-blocking

## What changes were proposed in this pull request?

StandaloneSchedulerBackend.executorRemoved is a blocking call right now. It may cause some deadlock since it's called inside StandaloneAppClient.ClientEndpoint.

This PR just changed CoarseGrainedSchedulerBackend.removeExecutor to be non-blocking. It's safe since the only two usages (StandaloneSchedulerBackend and YarnSchedulerEndpoint) don't need the return value).

## How was this patch tested?

Jenkins unit tests.

Author: Shixiong Zhu <shixiong@databricks.com>

Closes #14882 from zsxwing/SPARK-17316.

core/src/main/scala/org/apache/spark/scheduler/cluster/CoarseGrainedSchedulerBackend.scala

@@ -406,14 +406,15 @@ class CoarseGrainedSchedulerBackend(scheduler: TaskSchedulerImpl, val rpcEnv: Rp
     conf.getInt("spark.default.parallelism", math.max(totalCoreCount.get(), 2))
   }
 
-  // Called by subclasses when notified of a lost worker
-  def removeExecutor(executorId: String, reason: ExecutorLossReason) {
-    try {
-      driverEndpoint.askWithRetry[Boolean](RemoveExecutor(executorId, reason))
-    } catch {
-      case e: Exception =>
-        throw new SparkException("Error notifying standalone scheduler's driver endpoint", e)
-    }
+  /**
+   * Called by subclasses when notified of a lost worker. It just fires the message and returns
+   * at once.
+   */
+  protected def removeExecutor(executorId: String, reason: ExecutorLossReason): Unit = {
+    // Only log the failure since we don't care about the result.
+    driverEndpoint.ask(RemoveExecutor(executorId, reason)).onFailure { case t =>
+      logError(t.getMessage, t)
+    }(ThreadUtils.sameThread)
   }
 
   def sufficientResourcesRegistered(): Boolean = true