OF-709: Update Jetty from 7.4 to 9.1.5

I have reverted the [initial merge of the jetty update](https://github.com/igniterealtime/Openfire/pull/25), since it was merged without the approval at at least another developer. It was also a bundle of commits that should have been squashed before merging.

Here is the jetty update in a sane form for review.