fix: Jetty server becomes unresponsive after being idle

fix gravitee-io/issues#2549

pom.xml

@@ -47,7 +47,7 @@
         <apacheds-server-jndi.version>1.5.5</apacheds-server-jndi.version>
         <javax.servlet-api.version>3.1.0</javax.servlet-api.version>
         <jersey.version>2.29</jersey.version>
-        <jetty.version>9.4.17.v20190418</jetty.version>
+        <jetty.version>9.4.20.v20190813</jetty.version>
         <spring.security.version>5.1.5.RELEASE</spring.security.version>
         <validation-api.version>2.0.1.Final</validation-api.version>
         <mail.version>1.4.7</mail.version>