[SPARK-14513][CORE] Fix threads left behind after stopping SparkContext

## What changes were proposed in this pull request?

Shutting down `QueuedThreadPool` used by Jetty `Server` to avoid threads leakage after SparkContext is stopped.

Note: If this fix is going to apply to the `branch-1.6`, one more patch on the `NettyRpcEnv` class is needed so that the `NettyRpcEnv._fileServer.shutdown` is called in the `NettyRpcEnv.cleanup` method. This is due to the removal of `_fileServer` field in the `NettyRpcEnv` class in the master branch. Please advice if a second PR is necessary for bring this fix back to `branch-1.6`

## How was this patch tested?

Ran the ./dev/run-tests locally

Author: Terence Yim <terence@cask.co>

Closes #12318 from chtyim/fixes/SPARK-14513-thread-leak.

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

@@ -25,6 +25,7 @@ import org.eclipse.jetty.server.Server
 import org.eclipse.jetty.server.bio.SocketConnector
 import org.eclipse.jetty.server.ssl.SslSocketConnector
 import org.eclipse.jetty.servlet.{DefaultServlet, ServletContextHandler, ServletHolder}
+import org.eclipse.jetty.util.component.LifeCycle
 import org.eclipse.jetty.util.security.{Constraint, Password}
 import org.eclipse.jetty.util.thread.QueuedThreadPool
 
@@ -155,6 +156,12 @@ private[spark] class HttpServer(
       throw new ServerStateException("Server is already stopped")
     } else {
       server.stop()
+      // Stop the ThreadPool if it supports stop() method (through LifeCycle).
+      // It is needed because stopping the Server won't stop the ThreadPool it uses.
+      val threadPool = server.getThreadPool
+      if (threadPool != null && threadPool.isInstanceOf[LifeCycle]) {
+        threadPool.asInstanceOf[LifeCycle].stop
+      }
       port = -1
       server = null
     }

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

@@ -30,6 +30,7 @@ import org.eclipse.jetty.server.handler._
 import org.eclipse.jetty.server.nio.SelectChannelConnector
 import org.eclipse.jetty.server.ssl.SslSelectChannelConnector
 import org.eclipse.jetty.servlet._
+import org.eclipse.jetty.util.component.LifeCycle
 import org.eclipse.jetty.util.thread.QueuedThreadPool
 import org.json4s.JValue
 import org.json4s.jackson.JsonMethods.{pretty, render}
@@ -350,4 +351,15 @@ private[spark] object JettyUtils extends Logging {
 private[spark] case class ServerInfo(
     server: Server,
     boundPort: Int,
-    rootHandler: ContextHandlerCollection)
+    rootHandler: ContextHandlerCollection) {
+
+  def stop(): Unit = {
+    server.stop()
+    // Stop the ThreadPool if it supports stop() method (through LifeCycle).
+    // It is needed because stopping the Server won't stop the ThreadPool it uses.
+    val threadPool = server.getThreadPool
+    if (threadPool != null && threadPool.isInstanceOf[LifeCycle]) {
+      threadPool.asInstanceOf[LifeCycle].stop
+    }
+  }
+}

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

@@ -153,7 +153,7 @@ private[spark] abstract class WebUI(
   def stop() {
     assert(serverInfo.isDefined,
       "Attempted to stop %s before binding to a server!".format(className))
-    serverInfo.get.server.stop()
+    serverInfo.get.stop()
   }
 }