[SPARK-13501] Remove use of Guava Stopwatch

Our nightly doc snapshot builds are failing due to some issue involving the Guava Stopwatch constructor:

```
[error] /home/jenkins/workspace/spark-master-docs/spark/core/src/main/scala/org/apache/spark/scheduler/cluster/mesos/CoarseMesosSchedulerBackend.scala:496: constructor Stopwatch in class Stopwatch cannot be accessed in class CoarseMesosSchedulerBackend
[error]     val stopwatch = new Stopwatch()
[error]                     ^
```

This Stopwatch constructor was deprecated in newer versions of Guava (https://github.com/google/guava/commit/fd0cbc2c5c90e85fb22c8e86ea19630032090943) and it's possible that some classpath issues affecting Unidoc could be causing this to trigger compilation failures.

In order to work around this issue, this patch removes this use of Stopwatch since we don't use it anywhere else in the Spark codebase.

Author: Josh Rosen <joshrosen@databricks.com>

Closes #11376 from JoshRosen/remove-stopwatch.

core/src/main/scala/org/apache/spark/scheduler/cluster/mesos/CoarseMesosSchedulerBackend.scala

@@ -19,14 +19,12 @@ package org.apache.spark.scheduler.cluster.mesos
 
 import java.io.File
 import java.util.{Collections, List => JList}
-import java.util.concurrent.TimeUnit
 import java.util.concurrent.locks.ReentrantLock
 
 import scala.collection.JavaConverters._
 import scala.collection.mutable
 import scala.collection.mutable.{Buffer, HashMap, HashSet}
 
-import com.google.common.base.Stopwatch
 import org.apache.mesos.{Scheduler => MScheduler, SchedulerDriver}
 import org.apache.mesos.Protos.{TaskInfo => MesosTaskInfo, _}
 
@@ -493,12 +491,11 @@ private[spark] class CoarseMesosSchedulerBackend(
 
     // Wait for executors to report done, or else mesosDriver.stop() will forcefully kill them.
     // See SPARK-12330
-    val stopwatch = new Stopwatch()
-    stopwatch.start()
+    val startTime = System.nanoTime()
 
     // slaveIdsWithExecutors has no memory barrier, so this is eventually consistent
     while (numExecutors() > 0 &&
-      stopwatch.elapsed(TimeUnit.MILLISECONDS) < shutdownTimeoutMS) {
+      System.nanoTime() - startTime < shutdownTimeoutMS * 1000L * 1000L) {
       Thread.sleep(100)
     }