add atomic SetNX cache operation

Split the memory and Redis cache implementations into separate files and
provide backend-native atomic set-if-absent support.

management/server/cache/memory.go

@@ -0,0 +1,31 @@
+package cache
+
+import (
+	"context"
+	"time"
+
+	"github.com/eko/gocache/lib/v4/store"
+	gocachestore "github.com/eko/gocache/store/go_cache/v4"
+	gocache "github.com/patrickmn/go-cache"
+)
+
+type goCacheStore struct {
+	store.StoreInterface
+	client *gocache.Cache
+}
+
+func newMemoryStore(maxTimeout, cleanupInterval time.Duration) Store {
+	client := gocache.New(maxTimeout, cleanupInterval)
+	return &goCacheStore{
+		StoreInterface: gocachestore.NewGoCache(client),
+		client:         client,
+	}
+}
+
+func (s *goCacheStore) SetNX(_ context.Context, key, value string, ttl time.Duration) (bool, error) {
+	// Add only returns an error when a non-expired entry already exists.
+	if err := s.client.Add(key, value, ttl); err != nil {
+		return false, nil
+	}
+	return true, nil
+}