Upgrade Jetty to 9.4.8

Jetty 9.4.x was released over a year back : https://dev.eclipse.org/mhonarc/lists/jetty-announce/msg00097.html . Solr doesn't use any of the major improvements listed on the announce thread but it's the version that's in active development.

We should upgrade to Jetty 9.4.x series from 9.3.x

The latest version right now is 9.4.8.v20171121 . Upgrading it locally required a few compile time changes only.

Under "Default Sessions" in https://www.eclipse.org/jetty/documentation/9.4.x/upgrading-jetty.html#_upgrading_from_jetty_9_3_x_to_jetty_9_4_0 it states that "In previous versions of Jetty this was referred to as "hash" session management." .

The patch fixes all the compile time issues.
Currently two tests are failing:
TestRestManager
TestManagedSynonymGraphFilterFactory

Steps to upgrade the Jetty version were :
1. Modify ivy-versions.properties to reflect the new version number
2. Run ant jar-checksums to generate new JAR checksums