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/healther.go

@@ -0,0 +1,42 @@
+package health
+
+// Healther interface needs to be implemented by each middleware willing to
+// provide healthhceck information to the health middleware. As a second step
+// the middleware needs to registered against the health middleware, by addding
+// it to healthers map. Note this method should return quickly, i.e. just
+// checking a boolean status, as it is called every second from the health
+// middleware.
+type Healther interface {
+	// Health returns a boolean indicating the health status of a middleware.
+	// False indicates unhealthy.
+	Health() bool
+}
+
+// Ok returns the global health status of all middleware configured in this server.
+func (h *health) Ok() bool {
+	h.RLock()
+	defer h.RUnlock()
+	return h.ok
+}
+
+// SetOk sets the global health status of all middleware configured in this server.
+func (h *health) SetOk(ok bool) {
+	h.Lock()
+	defer h.Unlock()
+	h.ok = ok
+}
+
+// poll polls all healthers and sets the global state.
+func (h *health) poll() {
+	for _, m := range h.h {
+		if !m.Health() {
+			h.SetOk(false)
+			return
+		}
+	}
+	h.SetOk(true)
+}
+
+// Middleware that implements the Healther interface.
+// TODO(miek): none yet.
+var healthers = map[string]bool{}