Add content and snapshot store references

Update database object to hold reference to the
data stores.

Signed-off-by: Derek McGowan <derek@mcgstyle.net>

metadata/content.go

@@ -25,10 +25,17 @@ type contentStore struct {
 // NewContentStore returns a namespaced content store using an existing
 // content store interface.
 func NewContentStore(db *DB, cs content.Store) content.Store {
-	return &contentStore{
-		Store: cs,
-		db:    db,
+	db.storeL.Lock()
+	defer db.storeL.Unlock()
+
+	if db.cs == nil {
+		db.cs = &contentStore{
+			Store: cs,
+			db:    db,
+		}
 	}
+	return db.cs
+
 }
 
 func (cs *contentStore) Info(ctx context.Context, dgst digest.Digest) (content.Info, error) {