[nspcc-dev#112] api: Add start-after to ListObjectsV2 cache

In case when we send a response from a cache

Signed-off-by: Angira Kekteeva <kira@nspcc.ru>

api/layer/object.go

@@ -282,6 +282,7 @@ func (n *layer) ListObjectsV2(ctx context.Context, p *ListObjectsParamsV2) (*Lis
 
 	if p.ContinuationToken != "" {
 		allObjects = n.cache.GetCache(p.ContinuationToken, cacheKey)
+		allObjects = trimStartAfter(p.StartAfter, allObjects)
 	}
 
 	if allObjects == nil {
@@ -369,3 +370,14 @@ func (n *layer) listSortedAllObjects(ctx context.Context, p allObjectParams) ([]
 
 	return objects, nil
 }
+
+func trimStartAfter(startAfter string, objects []*ObjectInfo) []*ObjectInfo {
+	if objects != nil && len(startAfter) != 0 && objects[0].Name <= startAfter {
+		for i, _ := range objects {
+			if objects[i].Name > startAfter {
+				return objects[i:]
+			}
+		}
+	}
+	return objects
+}