## 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.

(cherry picked from commit 88a536babf119b7e331d02aac5d52b57658803bf)
Signed-off-by: Wenchen Fan <wenchen@databricks.com>