Jetty 9.4

[Ian Young]

Jetty 9.4 is at RC2 now. I had a quick look through the "upgrading" section of its documentation:

https://www.eclipse.org/jetty/documentation/9.4.x/upgrading-jetty.html#_upgrading_from_jetty_9_3_to_jetty_9_4

It looks to me as if some (more) incompatible configuration changes have been made for 9.4. I think that implies that we will need a new idp-jetty-base branch, assuming that at some point we will need to adopt it for the Windows installer.

    -- Ian