Refactor caching in poolmgr

Move separate caches to one cache -- functionServiceCache.  It can be
looked up by function, can update atime by address, and can be deleted
by podname.  This removes the other caches.  Some of the concurrency
logic is still a bit hairy; it might be better not to use fission.Cache.

poolmgr/poolmgr.go

@@ -56,9 +56,11 @@ func StartPoolmgr(controllerUrl string, namespace string, port int) error {
 		return err
 	}
 
-	gpm := MakeGenericPoolManager(controllerUrl, kubernetesClient, namespace)
+	fsCache := MakeFunctionServiceCache()
+
+	gpm := MakeGenericPoolManager(controllerUrl, kubernetesClient, namespace, fsCache)
+	api := MakeAPI(gpm, controllerClient, fsCache)
 
-	api := MakeAPI(gpm, controllerClient)
 	go api.Serve(port)
 
 	return nil