Backport: Add force option to set endpoint

The new `force=yes` option closes all connections to the server that is
being put into maintenance mode. This will immediately close all open
connections to the server without allowing results to return.

server/core/resource.cc

@@ -774,6 +774,11 @@ HttpResponse cb_set_server(const HttpRequest& request)
         string errmsg;
         if (mxs::server_set_status(server, opt, &errmsg))
         {
+            if (status_is_in_maint(opt) && request.get_option(CN_FORCE) == CN_YES)
+            {
+                dcb_hangup_foreach(server);
+            }
+
             return HttpResponse(MHD_HTTP_NO_CONTENT);
         }
         else