Make middleware survive a restart (#142)

Make middleware that sets up a (http) handler survive a graceful
restart. We calls the middleware's Shutdown function(s). If restart
fails the Start function is called again.

* middleware/health: OK
* middleware/pprof: OK
* middleware/metrics: OK

All restart OK.

core/setup/pprof.go

@@ -27,6 +27,7 @@ func PProf(c *Controller) (middleware.Middleware, error) {
 	handler := &pprof.Handler{}
 	pprofOnce.Do(func() {
 		c.Startup = append(c.Startup, handler.Start)
+		c.Shutdown = append(c.Shutdown, handler.Shutdown)
 	})
 
 	return func(next middleware.Handler) middleware.Handler {