LIHADOOP-38304 Missing one patch which enables configurable total jetty threads count in SHS

RB=1388604
BUG=LIHADOOP-38304
G=superfriends-reviewers
R=fli,mshen,yezhou,edlu
A=edlu

Author: zhouyejoe

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

@@ -305,7 +305,9 @@ private[spark] object JettyUtils extends Logging {
     addFilters(handlers, conf)
 
     // Start the server first, with no connectors.
-    val pool = new QueuedThreadPool
+    // Jetty threads count is configurable, default value is 200
+    val maxThreadsCount = conf.getInt("spark.history.maxJettyThreadsCount", 200)
+    val pool = new QueuedThreadPool(maxThreadsCount)
     if (serverName.nonEmpty) {
       pool.setName(serverName)
     }