Have run_single_worker.rb setup signal traps

Instead of the run_single_worker.rb script calling `.start_worker` on
the Runner class, have it instantiate a new class, call
`#setup_sigterm_trap` on that instance, and then call start (effectively
`start_worker`, just adds the `#setup_sigterm_trap` call in the middle)

lib/workers/bin/run_single_worker.rb

@@ -72,7 +72,8 @@
            end
 
   begin
-    worker.class::Runner.start_worker(runner_options.merge(:guid => worker.guid))
+    runner_options[:guid] = worker.guid
+    worker.class::Runner.new(runner_options).tap(&:setup_sigterm_trap).start
   ensure
     worker.delete
   end