Change avaje-jex-jetty to use jetty-server from jetty-servlet. From extend HttpServlet -> extend AbstractHandler

[rbygrave]

This drops the need for 2 jetty dependencies - jetty-servlet and jetty-security
Session support adjusts to chain the underlying handler
contextPath() adjusted to return ""

Downsides, well, it isn't a HttpServlet now so use of standard http servlet filters is more painful (but people would generally be using jex filters anyway so ...).