[SPARK-21176][WEB UI] Limit number of selector threads for admin ui proxy servlets to 8

## What changes were proposed in this pull request?
Please see also https://issues.apache.org/jira/browse/SPARK-21176

This change limits the number of selector threads that jetty creates to maximum 8 per proxy servlet (Jetty default is number of processors / 2).
The newHttpClient for Jettys ProxyServlet class is overwritten to avoid the Jetty defaults (which are designed for high-performance http servers).
Once https://github.com/eclipse/jetty.project/issues/1643 is available, the code could be cleaned up to avoid the method override.

I really need this on v2.1.1 - what is the best way for a backport automatic merge works fine)? Shall I create another PR?

## How was this patch tested?
(Please explain how this patch was tested. E.g. unit tests, integration tests, manual tests)
The patch was tested manually on a Spark cluster with a head node that has 88 processors using JMX to verify that the number of selector threads is now limited to 8 per proxy.

gurvindersingh zsxwing can you please review the change?

Author: IngoSchuster <ingo.schuster@de.ibm.com>
Author: Ingo Schuster <ingo.schuster@de.ibm.com>

Closes #18437 from IngoSchuster/master.

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

@@ -26,6 +26,8 @@ import scala.language.implicitConversions
 import scala.xml.Node
 
 import org.eclipse.jetty.client.api.Response
+import org.eclipse.jetty.client.HttpClient
+import org.eclipse.jetty.client.http.HttpClientTransportOverHTTP
 import org.eclipse.jetty.proxy.ProxyServlet
 import org.eclipse.jetty.server._
 import org.eclipse.jetty.server.handler._
@@ -208,6 +210,16 @@ private[spark] object JettyUtils extends Logging {
         rewrittenURI.toString()
       }
 
+      override def newHttpClient(): HttpClient = {
+        // SPARK-21176: Use the Jetty logic to calculate the number of selector threads (#CPUs/2),
+        // but limit it to 8 max.
+        // Otherwise, it might happen that we exhaust the threadpool since in reverse proxy mode
+        // a proxy is instantiated for each executor. If the head node has many processors, this
+        // can quickly add up to an unreasonably high number of threads.
+        val numSelectors = math.max(1, math.min(8, Runtime.getRuntime().availableProcessors() / 2))
+        new HttpClient(new HttpClientTransportOverHTTP(numSelectors), null)
+      }
+
       override def filterServerResponseHeader(
           clientRequest: HttpServletRequest,
           serverResponse: Response,