mw/health: poll other middleware (#976)

This add the infrastructure to let other middleware report their health
status back to the health middleware. A health.Healther interface is
introduced and a middleware needs to implement that. A middleware
that supports healthchecks is statically configured.

Every second each supported middleware is queried and the global health
state is updated.

Actual tests have been disabled as no other middleware implements this
at the moment.

middleware/health/health.go

@@ -16,6 +16,11 @@ type health struct {
 
 	ln  net.Listener
 	mux *http.ServeMux
+
+	// A slice of Healthers that the health middleware will poll every second for their health status.
+	h []Healther
+	sync.RWMutex
+	ok bool // ok is the global boolean indicating an all healthy middleware stack
 }
 
 func (h *health) Startup() error {
@@ -35,7 +40,12 @@ func (h *health) Startup() error {
 		h.mux = http.NewServeMux()
 
 		h.mux.HandleFunc(path, func(w http.ResponseWriter, r *http.Request) {
-			io.WriteString(w, ok)
+			if h.Ok() {
+				w.WriteHeader(http.StatusOK)
+				io.WriteString(w, ok)
+				return
+			}
+			w.WriteHeader(http.StatusServiceUnavailable)
 		})
 
 		go func() {