Invalidate stale router cache entry with podIP's for deleted pods. (#546)

The router's cache entry for a function might become stale if the pod that had the function specialized gets deleted somehow. In such a case, we'd retry getting a new service for the function from executor and retry forwarding the user request to the newly created service.

router/functionServiceMap.go

@@ -75,3 +75,8 @@ func (fmap *functionServiceMap) assign(f *metav1.ObjectMeta, serviceUrl *url.URL
 		// ignore error
 	}
 }
+
+func (fmap *functionServiceMap) remove(f *metav1.ObjectMeta) error {
+	mk := keyFromMetadata(f)
+	return fmap.cache.Delete(*mk)
+}