Upgrade to Jetty 9.2.21.v20170120

Release notes:

https://dev.eclipse.org/mhonarc/lists/jetty-announce/msg00099.html
https://dev.eclipse.org/mhonarc/lists/jetty-announce/msg00100.html

pom.xml

@@ -354,7 +354,7 @@
     <dependency>
       <groupId>org.eclipse.jetty</groupId>
       <artifactId>jetty-servlet</artifactId>
-      <version>9.2.15.v20160210</version>
+      <version>9.2.21.v20170120</version>
     </dependency>
     <dependency>
       <groupId>org.slf4j</groupId>