Use a daemon thread pool for REST server

The motivation is to fix failing tests SparkSubmitSuite and
DriverSuite.

Author: Andrew Or

core/src/main/scala/org/apache/spark/deploy/rest/SubmitRestServer.scala

@@ -26,6 +26,7 @@ import scala.io.Source
 import com.google.common.base.Charsets
 import org.eclipse.jetty.server.{Request, Server}
 import org.eclipse.jetty.server.handler.AbstractHandler
+import org.eclipse.jetty.util.thread.QueuedThreadPool
 
 import org.apache.spark.{SPARK_VERSION => sparkVersion, Logging, SparkConf}
 import org.apache.spark.util.Utils
@@ -52,6 +53,9 @@ private[spark] abstract class SubmitRestServer(host: String, requestedPort: Int,
 
   private def doStart(startPort: Int): (Server, Int) = {
     val server = new Server(new InetSocketAddress(host, requestedPort))
+    val threadPool = new QueuedThreadPool
+    threadPool.setDaemon(true)
+    server.setThreadPool(threadPool)
     server.setHandler(handler)
     server.start()
     val boundPort = server.getConnectors()(0).getLocalPort

core/src/test/scala/org/apache/spark/deploy/SparkSubmitSuite.scala

@@ -264,7 +264,6 @@ class SparkSubmitSuite extends FunSuite with Matchers with ResetSystemProperties
       "--name", "testApp",
       "--master", "local",
       "--conf", "spark.ui.enabled=false",
-      "--conf", "spark.master.rest.enabled=false",
       unusedJar.toString)
     runSparkSubmit(args)
   }
@@ -280,7 +279,6 @@ class SparkSubmitSuite extends FunSuite with Matchers with ResetSystemProperties
       "--master", "local-cluster[2,1,512]",
       "--jars", jarsString,
       "--conf", "spark.ui.enabled=false",
-      "--conf", "spark.master.rest.enabled=false",
       unusedJar.toString)
     runSparkSubmit(args)
   }