Jetty9 API Changes

These are the changes to the Jetty9 Webservice API that are being made to make this service consistent with the API of the web-routing service. 
Wasn't sure if we needed to take precautions to have a default empty `:context-listeners`.  Was thinking that the existing implementation would handle a `nil` value for this, in which case whatever `(:context-listeners options)` returns could just be passed through to `add-context-handler` without the intermediate map.  Maybe this is a `register-endpoint` restriction?